diff --git a/Exception.php b/Exception.php
deleted file mode 100644
index 623a8e1..0000000
--- a/Exception.php
+++ /dev/null
@@ -1,12 +0,0 @@
-<?php
-
-namespace SumoCoders\DeFactuur;
-
-/**
- * DeFactuur Exception class
- *
- * @author	Tijs Verkoyen <php-factr@verkoyen.eu>
- */
-class Exception extends \Exception
-{
-}
diff --git a/README.md b/README.md
index 137dec2..0027340 100755
--- a/README.md
+++ b/README.md
@@ -10,6 +10,41 @@ PHP DeFactuur is a (wrapper)class to communicate with [De Factuur](https://www.d
 
 PHP DeFactuur is [BSD](http://classes.verkoyen.eu/overview/bsd) licensed.
 
+## Initialisation
+
+Using symfony/httpclient:
+
+    use Nyholm\Psr7\Factory\Psr17Factory;
+    use Symfony\Component\HttpClient\Psr18Client;
+
+    $deFactuur = new DeFactuur(
+        new Psr18Client(),
+        new Psr17Factory(),
+        new Psr17Factory(),
+        'your_api_token'
+    );
+
+Using Guzzle:
+
+    $deFactuur = new \SumoCoders\DeFactuur\DeFactuur(
+        new \GuzzleHttp\Client(),
+        new \Nyholm\Psr7\Factory\Psr17Factory(),
+        new \Nyholm\Psr7\Factory\Psr17Factory(),    
+        'your_api_token'
+    );
+
+You can replace Psr17Factory with your own implementations of PSR-17's RequestFactoryInterface and StreamFactoryInterface.
+
+## Using DeFactuur as a Service
+
+Add the following to your services.yml:
+
+    SumoCoders\DeFactuur\DeFactuur:
+        arguments:
+            $apiToken: '%your.api.token%'
+
+If you use auto-wiring, that's it!
+
 ## Documentation
 
 Each method in the class is well documented using PHPDoc.
diff --git a/composer.json b/composer.json
index 361c608..65e7efa 100644
--- a/composer.json
+++ b/composer.json
@@ -15,7 +15,11 @@
 		"php": ">=7.4.0",
 		"ext-curl": "*",
 		"ext-json": "*",
-		"symfony/http-client": "^5.3"
+		"psr/http-client": "^1.0",
+		"nyholm/psr7": "^1.4"
+	},
+	"suggest": {
+		"symfony/http-client": "Low-level HTTP-client"
 	},
 	"autoload": {
 		"psr-4": {"SumoCoders\\DeFactuur\\": "src"}
diff --git a/src/Client/Address.php b/src/Client/Address.php
index 99c67ad..417e804 100644
--- a/src/Client/Address.php
+++ b/src/Client/Address.php
@@ -2,14 +2,6 @@
 
 namespace SumoCoders\DeFactuur\Client;
 
-/**
- * Address class
- *
- * @author		Tijs Verkoyen <php-factr@verkoyen.eu>
- * @version		2.0.0
- * @copyright	Copyright (c), Tijs Verkoyen. All rights reserved.
- * @license		BSD License
- */
 class Address
 {
     protected string $country;
diff --git a/src/Client/Client.php b/src/Client/Client.php
index 1784306..726e83d 100644
--- a/src/Client/Client.php
+++ b/src/Client/Client.php
@@ -2,59 +2,84 @@
 
 namespace SumoCoders\DeFactuur\Client;
 
-/**
- * Client class
- *
- * @author		Tijs Verkoyen <php-factr@verkoyen.eu>
- * @version		2.0.0
- * @copyright	Copyright (c), Tijs Verkoyen. All rights reserved.
- * @license		BSD License
- */
 class Client
 {
-    protected int $id;
+    // required
+    protected string $lastName;
 
-    protected int $paymentDays;
+    protected array $email;
 
-    protected int $replacedById;
+    protected Address $billingAddress;
 
-    protected string $cid;
+    protected Address $companyAddress;
 
-    protected string $company;
+    // optional
+    protected ?int $id;
 
-    protected string $vat;
+    protected ?int $paymentDays;
 
-    protected string $firstName;
+    protected ?int $replacedById;
 
-    protected string $lastName;
+    protected ?string $cid;
 
-    protected string $phone;
+    protected ?string $company;
 
-    protected string $fax;
+    protected ?string $vat;
 
-    protected string $cell;
+    protected ?string $firstName;
 
-    protected string $website;
+    protected ?string $phone;
 
-    protected string $remarks;
+    protected ?string $fax;
 
-    protected Address $billingAddress;
+    protected ?string $cell;
 
-    protected Address $companyAddress;
+    protected ?string $website;
 
-    protected array $email;
+    protected ?string $remarks;
 
-    protected bool $invoiceableByEmail;
+    protected ?bool $invoiceableByEmail;
 
-    protected bool $invoiceableBySnailMail;
+    protected ?bool $invoiceableBySnailMail;
 
-    protected bool $invoiceableByFactr;
+    protected ?bool $invoiceableByFactr;
 
-    protected bool $disabled = false;
+    protected ?bool $disabled;
 
-    public function setBillingAddress(Address $billingAddress): void
+    public function __construct(
+        string $lastName,
+        array $email,
+        Address $billingAddress,
+        Address $companyAddress
+    ) {
+        $this->lastName = $lastName;
+        $this->email = $email;
+        $this->billingAddress = $billingAddress;
+        $this->companyAddress = $companyAddress;
+
+        $this->id = null;
+        $this->paymentDays = null;
+        $this->replacedById = null;
+        $this->cid = null;
+        $this->company = null;
+        $this->vat = null;
+        $this->firstName = null;
+        $this->phone = null;
+        $this->fax = null;
+        $this->cell = null;
+        $this->website = null;
+        $this->remarks = null;
+        $this->invoiceableByEmail = null;
+        $this->invoiceableBySnailMail = null;
+        $this->invoiceableByFactr = null;
+        $this->disabled = null;
+    }
+
+    public function setBillingAddress(Address $billingAddress): Client
     {
         $this->billingAddress = $billingAddress;
+
+        return $this;
     }
 
     public function getBillingAddress(): Address
@@ -62,32 +87,38 @@ public function getBillingAddress(): Address
         return $this->billingAddress;
     }
 
-    public function setCell(string $cell): void
+    public function setCell(string $cell): Client
     {
         $this->cell = $cell;
+
+        return $this;
     }
 
-    public function getCell(): string
+    public function getCell(): ?string
     {
         return $this->cell;
     }
 
-    public function setCid(string $cid): void
+    public function setCid(string $cid): Client
     {
         $this->cid = $cid;
+
+        return $this;
     }
 
-    public function getCid(): string
+    public function getCid(): ?string
     {
         return $this->cid;
     }
 
-    public function setCompany(string $company): void
+    public function setCompany(string $company): Client
     {
         $this->company = $company;
+
+        return $this;
     }
 
-    public function getCompany(): string
+    public function getCompany(): ?string
     {
         return $this->company;
     }
@@ -102,14 +133,18 @@ public function getCompanyAddress(): Address
         return $this->companyAddress;
     }
 
-    public function addEmail(string $email): void
+    public function addEmail(string $email): Client
     {
         $this->email[] = $email;
+
+        return $this;
     }
 
-    public function setEmail(array $email): void
+    public function setEmail(array $email): Client
     {
         $this->email = $email;
+
+        return $this;
     }
 
     public function getEmail(): array
@@ -117,69 +152,83 @@ public function getEmail(): array
         return $this->email;
     }
 
-    public function setFax(string $fax): void
+    public function setFax(string $fax): Client
     {
         $this->fax = $fax;
+
+        return $this;
     }
 
-    public function getFax(): string
+    public function getFax(): ?string
     {
         return $this->fax;
     }
 
-    public function setFirstName(string $firstName): void
+    public function setFirstName(string $firstName): Client
     {
         $this->firstName = $firstName;
+
+        return $this;
     }
 
-    public function getFirstName(): string
+    public function getFirstName(): ?string
     {
         return $this->firstName;
     }
 
-    private function setId(int $id): void
+    private function setId(int $id): Client
     {
         $this->id = $id;
+
+        return $this;
     }
 
-    public function getId(): int
+    public function getId(): ?int
     {
         return $this->id;
     }
 
-    public function setInvoiceableByEmail(bool $invoiceableByEmail): void
+    public function setInvoiceableByEmail(bool $invoiceableByEmail): Client
     {
         $this->invoiceableByEmail = $invoiceableByEmail;
+
+        return $this;
     }
 
-    public function getInvoiceableByEmail(): bool
+    public function getInvoiceableByEmail(): ?bool
     {
         return $this->invoiceableByEmail;
     }
 
-    public function setInvoiceableByFactr(bool $invoiceableByFactr): void
+    public function setInvoiceableByFactr(bool $invoiceableByFactr): Client
     {
         $this->invoiceableByFactr = $invoiceableByFactr;
+
+        return $this;
     }
 
-    public function getInvoiceableByFactr(): bool
+    public function getInvoiceableByFactr(): ?bool
     {
         return $this->invoiceableByFactr;
     }
 
-    public function setInvoiceableBySnailMail(bool $invoiceableBySnailMail): void
+    public function setInvoiceableBySnailMail(bool $invoiceableBySnailMail): Client
     {
         $this->invoiceableBySnailMail = $invoiceableBySnailMail;
+
+        return $this;
     }
 
-    public function getInvoiceableBySnailMail(): bool
+    public function getInvoiceableBySnailMail(): ?bool
     {
         return $this->invoiceableBySnailMail;
     }
 
-    public function setLastName(string $lastName): void
+    public function setLastName(string $lastName): Client
     {
         $this->lastName = $lastName;
+
+        return $this;
     }
 
     public function getLastName(): string
@@ -187,74 +236,88 @@ public function getLastName(): string
         return $this->lastName;
     }
 
-    public function setPaymentDays(int $paymentDays): void
+    public function setPaymentDays(int $paymentDays): Client
     {
         $this->paymentDays = $paymentDays;
+
+        return $this;
     }
 
-    public function getPaymentDays(): int
+    public function getPaymentDays(): ?int
     {
         return $this->paymentDays;
     }
 
-    public function setPhone(string $phone): void
+    public function setPhone(string $phone): Client
     {
         $this->phone = $phone;
+
+        return $this;
     }
 
-    public function getPhone(): string
+    public function getPhone(): ?string
     {
         return $this->phone;
     }
 
-    public function setRemarks(string $remarks): void
+    public function setRemarks(string $remarks): Client
     {
         $this->remarks = $remarks;
+
+        return $this;
     }
 
-    public function getRemarks(): string
+    public function getRemarks(): ?string
     {
         return $this->remarks;
     }
 
-    public function setVat(string $vat): void
+    public function setVat(string $vat): Client
     {
         $this->vat = $vat;
+
+        return $this;
     }
 
-    public function getVat(): string
+    public function getVat(): ?string
     {
         return $this->vat;
     }
 
-    public function setWebsite(string $website): void
+    public function setWebsite(string $website): Client
     {
         $this->website = $website;
+
+        return $this;
     }
 
-    public function getWebsite(): string
+    public function getWebsite(): ?string
     {
         return $this->website;
     }
 
-    public function getReplacedById(): int
+    public function getReplacedById(): ?int
     {
         return $this->replacedById;
     }
 
-    public function setReplacedById(int $replacedById): void
+    public function setReplacedById(int $replacedById): Client
     {
         $this->replacedById = $replacedById;
+
+        return $this;
     }
 
-    public function isDisabled(): bool
+    public function isDisabled(): ?bool
     {
         return $this->disabled;
     }
 
-    public function setDisabled(bool $disabled = true): void
+    public function setDisabled(bool $disabled = true): Client
     {
         $this->disabled = $disabled;
+
+        return $this;
     }
 
     /**
@@ -262,7 +325,12 @@ public function setDisabled(bool $disabled = true): void
      */
     public static function initializeWithRawData(array $data): Client
     {
-        $item = new Client();
+        $item = new Client(
+            $data['last_name'] ?? '',
+            $data['email'] ?? '',
+            Address::initializeWithRawData($data['billing_address'] ?? []),
+            Address::initializeWithRawData($data['company_address'] ?? [])
+        );
 
         if(isset($data['id'])) $item->setId($data['id']);
         if(isset($data['cid'])) $item->setCid($data['cid']);
@@ -300,7 +368,7 @@ public static function initializeWithRawData(array $data): Client
      */
     public function toArray(bool $forApi = false): array
     {
-        $data = array();
+        $data = [];
 
         $data['id'] = $this->getId();
         $data['cid'] = $this->getCid();
diff --git a/DeFactuur.php b/src/DeFactuur.php
similarity index 77%
rename from DeFactuur.php
rename to src/DeFactuur.php
index 4f60f8f..6247721 100644
--- a/DeFactuur.php
+++ b/src/DeFactuur.php
@@ -3,7 +3,11 @@
 namespace SumoCoders\DeFactuur;
 
 use InvalidArgumentException;
-use SumoCoders\DeFactuur\Exception as FactuurException;
+use Psr\Http\Client\ClientExceptionInterface;
+use Psr\Http\Client\ClientInterface;
+use Psr\Http\Message\RequestFactoryInterface;
+use Psr\Http\Message\StreamFactoryInterface;
+use SumoCoders\DeFactuur\Exception as DeFactuurException;
 use SumoCoders\DeFactuur\Client\Client;
 use SumoCoders\DeFactuur\Invoice\Invoice;
 use SumoCoders\DeFactuur\Invoice\Mail;
@@ -11,14 +15,6 @@
 use SumoCoders\DeFactuur\Product\Product;
 use Exception;
 
-/**
- * DeFactuur class
- *
- * @author		Tijs Verkoyen <php-factr@verkoyen.eu>
- * @version		2.0.0
- * @copyright	Copyright (c), Tijs Verkoyen. All rights reserved.
- * @license		BSD License
- */
 class DeFactuur
 {
     // internal constant to enable/disable debugging
@@ -41,12 +37,11 @@ class DeFactuur
      */
     private string $apiToken;
 
-    /**
-     * cURL instance
-     *
-     * @var	resource
-     */
-    private $curl;
+    private ClientInterface $client;
+
+    private RequestFactoryInterface $requestFactory;
+
+    private StreamFactoryInterface $streamFactory;
 
     /**
      * The timeout
@@ -59,13 +54,19 @@ class DeFactuur
     private string $userAgent;
 
 // class methods
-    /**
-     * Default destructor
-     */
-    public function __destruct()
-    {
-        // close the curl-instance if needed
-        if($this->curl != null) curl_close($this->curl);
+    public function __construct(
+        ClientInterface $httpClient,
+        RequestFactoryInterface $requestFactory,
+        StreamFactoryInterface $streamFactory,
+        ?string $apiToken = null
+    ) {
+        $this->client = $httpClient;
+        $this->requestFactory = $requestFactory;
+        $this->streamFactory = $streamFactory;
+
+        if ($apiToken !== null && $apiToken !== '') {
+            $this->apiToken = $apiToken;
+        }
     }
 
     /**
@@ -110,7 +111,7 @@ private function encodeData($data, array $array = [], ?string $prefix = null): a
      * Make the call
      *
      * @return array|bool|string
-     * @throws FactuurException
+     * @throws DeFactuurException
      */
     private function doCall(
         string $url,
@@ -118,19 +119,13 @@ private function doCall(
         string $method = 'GET',
         bool $returnHeaders = false
     ) {
-        // redefine
-        $options = array();
+        $data = null;
 
         // add credentials
         $parameters['api_key'] = $this->getApiToken();
 
         // through GET
         if ($method == 'GET') {
-            // remove POST-specific stuff
-            unset($options[CURLOPT_HTTPHEADER]);
-            unset($options[CURLOPT_POST]);
-            unset($options[CURLOPT_POSTFIELDS]);
-
             // build url
             $url .= '?' . http_build_query($parameters, null);
             $url = $this->removeIndexFromArrayParameters($url);
@@ -141,86 +136,74 @@ private function doCall(
                 $data = http_build_query($data, null);
                 $data = $this->removeIndexFromArrayParameters($data);
             }
-
-            $options[CURLOPT_POST] = true;
-            $options[CURLOPT_POSTFIELDS] = $data;
         } elseif ($method == 'DELETE') {
-            unset($options[CURLOPT_HTTPHEADER]);
-            unset($options[CURLOPT_POST]);
-            unset($options[CURLOPT_POSTFIELDS]);
-            $options[CURLOPT_CUSTOMREQUEST] = 'DELETE';
-
             // build url
             $url .= '?' . http_build_query($parameters, null);
         } elseif ($method == 'PUT') {
             $data = $this->encodeData($parameters);
             $data = http_build_query($data, null);
             $data = $this->removeIndexFromArrayParameters($data);
-
-            $options[CURLOPT_POSTFIELDS] = $data;
-            $options[CURLOPT_CUSTOMREQUEST] = 'PUT';
-        } else throw new FactuurException('Unsupported method (' . $method . ')');
+        } else throw new DeFactuurException('Unsupported method (' . $method . ')');
 
         // prepend
         $url = self::API_URL . '/' . self::API_VERSION . '/' . $url;
+        $request = $this->requestFactory->createRequest($method, $url);
 
-        // set options
-        $this->getCurlOptions($url);
-
-        // init
-        $this->curl = curl_init();
-
-        // set options
-        curl_setopt_array($this->curl, $options);
-
-        // execute
-        $response = curl_exec($this->curl);
-        $headers = curl_getinfo($this->curl);
-
-        // fetch errors
-        $errorNumber = curl_errno($this->curl);
-        $errorMessage = curl_error($this->curl);
-
-        // check status code
-        if ($headers['http_code'] >= 400) {
-            // try to read JSON
-            $json = @json_decode($response, true);
+        if ($data !== null) {
+           $request = $request->withBody($this->streamFactory->createStream($data));
+        }
 
-            // throw
-            if ($json !== null && $json !== false) {
+        try {
+            $response = $this->client->sendRequest($request);
+        } catch (ClientExceptionInterface $e) {
+            throw new DeFactuurException($e->getMessage());
+        }
 
-                // errors?
-                if (is_array($json) && array_key_exists('errors', $json)) {
-                    $message = '';
-                    foreach($json['errors'] as $key => $value) $message .= $key . ': ' . implode(', ', $value) . "\n";
+        if ($response->getStatusCode() === 422) {
+            // Unprocessable entity = validation error on data that was passed
+            throw new DeFactuurException(
+                'Validation error - Unprocessable entity. (' . $response->getStatusCode() . ')',
+                $response->getStatusCode()
+            );
+        }
 
-                    throw new FactuurException(trim($message));
-                } else {
-                    if(is_array($json) && array_key_exists('message', $json)) $response = $json['message'];
-                    throw new FactuurException($response, $headers['http_code']);
+        if ($response->getStatusCode() >= 400) {
+            try {
+                $json = json_decode($response->getBody()->getContents(), true);
+
+                // throw
+                if ($json !== null && $json !== false) {
+                    // errors?
+                    if (is_array($json) && array_key_exists('errors', $json)) {
+                        $message = '';
+                        foreach ($json['errors'] as $key => $value) $message .= $key . ': ' . implode(', ', $value) . "\n";
+
+                        throw new DeFactuurException(trim($message));
+                    } else {
+                        if (is_array($json) && array_key_exists('message', $json)) $response = $json['message'];
+                        throw new DeFactuurException($response, $response->getStatusCode());
+                    }
                 }
+            } catch (\Exception $e) {
+                throw new DeFactuurException($e->getMessage());
             }
 
             // unknown error
-            throw new FactuurException('Invalid response (' . $headers['http_code'] . ')', $headers['http_code']);
+            throw new DeFactuurException('Invalid response (' . $response->getStatusCode() . ')', $response->getStatusCode());
         }
 
-        // error?
-        if($errorNumber != '') throw new FactuurException($errorMessage, $errorNumber);
-
         // return the headers if needed
-        if($returnHeaders) return $headers;
+        if($returnHeaders) return $response->getHeaders();
 
         if (stristr($url, '.pdf')) {
             // Return pdf contents immediately without tampering with them
-            return $response;
+            return $response->getBody()->getContents();
         }
 
-        // we expect JSON so decode it
-        $json = @json_decode($response, true);
+        $json = json_decode($response->getBody()->getContents(), true);
 
         // validate json
-        if($json === false) throw new FactuurException('Invalid JSON-response');
+        if($json === false) throw new DeFactuurException('Invalid JSON-response');
 
         // decode the response
         array_walk_recursive($json, array(__CLASS__, 'decodeResponse'));
@@ -317,64 +300,44 @@ public function setUserAgent(string $userAgent): void
     /**
      * Get an API token
      *
-     * @throws FactuurException
+     * @throws DeFactuurException
      */
     public function accountApiToken(string $username, string $password): string
     {
         $url = self::API_URL . '/' . self::API_VERSION . '/account/api_token.json';
 
-        // set options
-        $options = $this->getCurlOptions($url);
-        $options[CURLOPT_HTTPAUTH] = CURLAUTH_BASIC;
-        $options[CURLOPT_USERPWD] = $username . ':' . $password;
-
-        // init
-        $this->curl = curl_init();
+        $request = $this->requestFactory->createRequest('GET', $url);
+        $request = $request->withAddedHeader(
+            'Authorization',
+            'Basic ' . base64_encode($username . ':' . $password)
+        );
 
-        // set options
-        curl_setopt_array($this->curl, $options);
+        try {
+            $response = $this->client->sendRequest($request);
 
-        // execute
-        $response = curl_exec($this->curl);
-        $headers = curl_getinfo($this->curl);
+            // check status code
+            if ($response->getStatusCode() != 200) {
+                throw new DeFactuurException('Could\'t authenticate you');
+            }
 
-        // check status code
-        if ($headers['http_code'] != 200) {
-            throw new FactuurException('Could\'t authenticate you');
+            // we expect JSON so decode it
+            $json = json_decode($response->getBody()->getContents(), true);
+        } catch (ClientExceptionInterface $e) {
+            throw new DeFactuurException($e->getMessage());
         }
 
-        // we expect JSON so decode it
-        $json = @json_decode($response, true);
-
         // validate json
-        if($json === false || !isset($json['api_token'])) throw new FactuurException('Invalid JSON-response');
-
-        // set the token
-        $this->setApiToken($json['api_token']);
+        if($json === false || !isset($json['api_token'])) throw new DeFactuurException('Invalid JSON-response');
 
         // return
         return $json['api_token'];
     }
 
-    private function getCurlOptions(string $url): array
-    {
-        $options[CURLOPT_URL] = $url;
-        $options[CURLOPT_PORT] = self::API_PORT;
-        $options[CURLOPT_USERAGENT] = $this->getUserAgent();
-        $options[CURLOPT_FOLLOWLOCATION] = true;
-        $options[CURLOPT_SSL_VERIFYPEER] = false;
-        $options[CURLOPT_SSL_VERIFYHOST] = false;
-        $options[CURLOPT_RETURNTRANSFER] = true;
-        $options[CURLOPT_TIMEOUT] = $this->getTimeOut();
-
-        return $options;
-    }
-
 // client methods
     /**
      * Get a list of all the clients for the authenticating user.
      *
-     * @throws FactuurException
+     * @throws DeFactuurException
      */
     public function clients(): array
     {
@@ -393,7 +356,7 @@ public function clients(): array
      * Get all of the available information for a single client. You 'll need the id of the client.
      *
      * @return Client|bool
-     * @throws FactuurException
+     * @throws DeFactuurException
      */
     public function clientsGet(string $id)
     {
@@ -407,7 +370,7 @@ public function clientsGet(string $id)
      * Get all clients that are linked to an email address
      *
      * @return Client[]|bool
-     * @throws FactuurException
+     * @throws DeFactuurException
      */
     public function clientsGetByEmail(string $email)
     {
@@ -427,7 +390,7 @@ function (array $clientData) {
     /**
      * Create a new client.
      *
-     * @throws FactuurException
+     * @throws DeFactuurException
      */
     public function clientsCreate(Client $client): Client
     {
@@ -440,7 +403,7 @@ public function clientsCreate(Client $client): Client
     /**
      * Update an existing client
      *
-     * @throws FactuurException
+     * @throws DeFactuurException
      */
     public function clientsUpdate(string $id, Client $client): bool
     {
@@ -453,7 +416,7 @@ public function clientsUpdate(string $id, Client $client): bool
     /**
      * Check if country is European
      *
-     * @throws FactuurException
+     * @throws DeFactuurException
      */
     public function clientsIsEuropean(string $countryCode): bool
     {
@@ -466,7 +429,7 @@ public function clientsIsEuropean(string $countryCode): bool
     /**
      * Delete a client
      *
-     * @throws FactuurException
+     * @throws DeFactuurException
      */
     public function clientsDelete(string $id): bool
     {
@@ -478,7 +441,7 @@ public function clientsDelete(string $id): bool
     /**
      * Disable client in favour of another client
      *
-     * @throws FactuurException
+     * @throws DeFactuurException
      */
     public function clientsDisable(string $id, string $replacedById): bool
     {
@@ -569,7 +532,7 @@ public function invoicesGet(string $id)
      * Get the pdf for an invoice
      *
      * @return string|bool Raw PDF contents
-     * @throws FactuurException
+     * @throws DeFactuurException
      */
     public function invoicesGetAsPdf(string $id)
     {
@@ -630,7 +593,7 @@ public function invoicesCreateCreditNote(string $id, Invoice $creditNote): Invoi
     /**
      * Update an existing invoice
      *
-     * @throws FactuurException
+     * @throws DeFactuurException
      */
     public function invoicesUpdate(string $id, Invoice $invoice): bool
     {
@@ -643,7 +606,7 @@ public function invoicesUpdate(string $id, Invoice $invoice): bool
     /**
      * Delete an invoice
      *
-     * @throws FactuurException
+     * @throws DeFactuurException
      */
     public function invoicesDelete(int $id): bool
     {
@@ -656,7 +619,7 @@ public function invoicesDelete(int $id): bool
      * Sending an invoice by mail.
      *
      * @return Mail|bool
-     * @throws FactuurException
+     * @throws DeFactuurException
      */
     public function invoiceSendByMail(
         string $id,
@@ -681,7 +644,7 @@ public function invoiceSendByMail(
     /**
      * Marking invoice as sent by mail.
      *
-     * @throws FactuurException
+     * @throws DeFactuurException
      */
     public function invoiceMarkAsSentByMail(string $id, string $email): void
     {
@@ -709,7 +672,7 @@ public function invoicesAddPayment(string $id, Payment $payment): Payment
      *
      * @param string $id The invoice id
      *
-     * @throws FactuurException
+     * @throws DeFactuurException
      */
     public function invoiceSendReminder(string $id): array
     {
@@ -719,7 +682,7 @@ public function invoiceSendReminder(string $id): array
     /**
      * Check if vat is required
      *
-     * @throws FactuurException
+     * @throws DeFactuurException
      */
     public function invoicesVatRequired(string $countryCode, bool $isCompany): bool
     {
@@ -733,7 +696,7 @@ public function invoicesVatRequired(string $countryCode, bool $isCompany): bool
     /**
      * Check if valid vat number
      *
-     * @throws FactuurException
+     * @throws DeFactuurException
      */
     public function isValidVat(string $vatNumber): bool
     {
@@ -746,7 +709,7 @@ public function isValidVat(string $vatNumber): bool
     /**
      * Upload a CODA file and let DeFactuur interpret it
      *
-     * @throws FactuurException
+     * @throws DeFactuurException
      */
     public function uploadCodaFile(string $filePath): array
     {
@@ -759,7 +722,7 @@ public function uploadCodaFile(string $filePath): array
     }
 
     /**
-     * @throws FactuurException
+     * @throws DeFactuurException
      */
     public function products(): array
     {
@@ -777,7 +740,7 @@ public function products(): array
 
     /**
      * @return Product|bool
-     * @throws FactuurException
+     * @throws DeFactuurException
      */
     public function productsGet(int $id)
     {
@@ -791,7 +754,7 @@ public function productsGet(int $id)
     }
 
     /**
-     * @throws FactuurException
+     * @throws DeFactuurException
      */
     public function productsCreate(Product $product): Product
     {
diff --git a/src/Exception.php b/src/Exception.php
new file mode 100644
index 0000000..592db44
--- /dev/null
+++ b/src/Exception.php
@@ -0,0 +1,7 @@
+<?php
+
+namespace SumoCoders\DeFactuur;
+
+class Exception extends \Exception
+{
+}
diff --git a/src/Invoice/Invoice.php b/src/Invoice/Invoice.php
index 6a39f26..4988563 100644
--- a/src/Invoice/Invoice.php
+++ b/src/Invoice/Invoice.php
@@ -5,6 +5,7 @@
 use SumoCoders\DeFactuur\Client\Client;
 use DateTime;
 use Exception;
+use SumoCoders\DeFactuur\ValueObject\State;
 
 /**
  * Class Invoice
@@ -13,57 +14,86 @@
  */
 class Invoice
 {
-    protected int $id;
+    protected ?Client $client;
 
-    protected int $clientId;
+    protected ?int $clientId;
 
-    protected string $iid;
+    /** @var Item[] */
+    protected array $items;
 
-    protected string $state;
+    protected ?int $id;
 
-    protected string $paymentMethod;
+    protected ?string $iid;
 
-    protected string $description;
+    protected ?State $state;
 
-    protected string $shownRemark;
+    protected ?string $paymentMethod;
 
-    protected DateTime $generated;
+    protected ?string $description;
 
-    protected DateTime $dueDate;
+    protected ?string $shownRemark;
 
-    protected DateTime $fullyPaidAt;
+    protected ?DateTime $generated;
 
-    /** @var Item[] */
-    protected array $items;
+    protected ?DateTime $dueDate;
+
+    protected ?DateTime $fullyPaidAt;
 
     /** @var Payment[] */
-    protected array $payments;
+    protected ?array $payments;
 
     /** @var History[] */
-    protected array $history;
+    protected ?array $history;
 
-    protected float $total;
+    protected ?float $total;
 
-    protected Client $client;
+    protected ?string $language;
 
-    protected string $language;
-
-    protected float $discount;
+    protected ?float $discount;
 
     protected bool $discountIsPercentage = false;
 
-    protected string $discountDescription;
+    protected ?string $discountDescription;
 
-    protected string $vatException;
+    protected ?string $vatException;
 
-    protected string $vatDescription;
+    protected ?string $vatDescription;
 
     protected bool $prepareForSending = false;
 
-    public function setClient(Client $client): void
+    public function __construct(
+        ?Client $client = null,
+        array $items = []
+    ) {
+        $this->client = $client;
+        $this->clientId = $client === null ? null : $client->getId();
+        $this->items = $items;
+
+        $this->id = null;
+        $this->iid = null;
+        $this->state = null;
+        $this->paymentMethod = null;
+        $this->description = null;
+        $this->shownRemark = null;
+        $this->generated = null;
+        $this->dueDate = null;
+        $this->fullyPaidAt = null;
+        $this->payments = null;
+        $this->history = null;
+        $this->total = null;
+        $this->language = null;
+        $this->discount = null;
+        $this->discountDescription = null;
+        $this->vatException = null;
+        $this->vatDescription = null;
+    }
+
+    public function setClient(Client $client): Invoice
     {
         $this->client = $client;
-        $this->setClientId($client->getId());
+        $this->clientId = $client->getId();
+
+        return $this;
     }
 
     public function getClient(): Client
@@ -71,9 +101,11 @@ public function getClient(): Client
         return $this->client;
     }
 
-    public function setClientId(int $clientId): void
+    public function setClientId(int $clientId): Invoice
     {
         $this->clientId = $clientId;
+
+        return $this;
     }
 
     public function getClientId(): int
@@ -81,12 +113,14 @@ public function getClientId(): int
         return $this->clientId;
     }
 
-    public function setDescription(string $description): void
+    public function setDescription(string $description): Invoice
     {
         $this->description = $description;
+
+        return $this;
     }
 
-    public function getDescription(): string
+    public function getDescription(): ?string
     {
         return $this->description;
     }
@@ -96,7 +130,7 @@ private function setDueDate(DateTime $dueDate): void
         $this->dueDate = $dueDate;
     }
 
-    public function getDueDate(): DateTime
+    public function getDueDate(): ?DateTime
     {
         return $this->dueDate;
     }
@@ -106,7 +140,7 @@ private function setFullyPaidAt(DateTime $fullyPaidAt): void
         $this->fullyPaidAt = $fullyPaidAt;
     }
 
-    public function getFullyPaidAt(): DateTime
+    public function getFullyPaidAt(): ?DateTime
     {
         return $this->fullyPaidAt;
     }
@@ -116,7 +150,7 @@ private function setGenerated(DateTime $generated): void
         $this->generated = $generated;
     }
 
-    public function getGenerated(): DateTime
+    public function getGenerated(): ?DateTime
     {
         return $this->generated;
     }
@@ -129,15 +163,17 @@ private function addHistory(History $history): void
     /**
      * @param History[] $history
      */
-    public function setHistory(array $history): void
+    public function setHistory(array $history): Invoice
     {
         $this->history = $history;
+
+        return $this;
     }
 
     /**
      * @return History[]
      */
-    public function getHistory(): array
+    public function getHistory(): ?array
     {
         return $this->history;
     }
@@ -147,7 +183,7 @@ private function setId(int $id): void
         $this->id = $id;
     }
 
-    public function getId(): int
+    public function getId(): ?int
     {
         return $this->id;
     }
@@ -157,22 +193,26 @@ private function setIid(string $iid): void
         $this->iid = $iid;
     }
 
-    public function getIid(): string
+    public function getIid(): ?string
     {
         return $this->iid;
     }
 
-    public function addItem(Item $item): void
+    public function addItem(Item $item): Invoice
     {
         $this->items[] = $item;
+
+        return $this;
     }
 
     /**
      * @param Item[] $items
      */
-    public function setItems(array $items): void
+    public function setItems(array $items): Invoice
     {
         $this->items = $items;
+
+        return $this;
     }
 
     /**
@@ -194,46 +234,43 @@ private function addPayment(Payment $payment): void
     /**
      * @return Payment[]
      */
-    public function getPayments(): array
+    public function getPayments(): ?array
     {
         return $this->payments;
     }
 
-    public function setShownRemark(string $shownRemark): void
+    public function setShownRemark(string $shownRemark): Invoice
     {
         $this->shownRemark = $shownRemark;
+
+        return $this;
     }
 
-    public function getShownRemark(): string
+    public function getShownRemark(): ?string
     {
         return $this->shownRemark;
     }
 
-    /**
-     * @param string $state Possible states are: paid, sent, created
-     * @todo: valueObject?
-     */
-    public function setState(string $state): void
+    public function setState(State $state): Invoice
     {
         $this->state = $state;
+
+        return $this;
     }
 
-    public function getState(): string
+    public function getState(): ?State
     {
         return $this->state;
     }
 
-    /**
-     * @param string $paymentMethod Possible payment methods are:
-     *  not_paid, cheque, transfer, bankcontact, cash, direct_debit and paid
-     * @todo: valueObject
-     */
-    public function setPaymentMethod(string $paymentMethod): void
+    public function setPaymentMethod(string $paymentMethod): Invoice
     {
         $this->paymentMethod = $paymentMethod;
+
+        return $this;
     }
 
-    public function getPaymentMethod(): string
+    public function getPaymentMethod(): ?string
     {
         return $this->paymentMethod;
     }
@@ -243,22 +280,24 @@ private function setTotal(float $total): void
         $this->total = $total;
     }
 
-    public function getTotal(): float
+    public function getTotal(): ?float
     {
         return $this->total;
     }
 
-    public function setLanguage(string $language): void
+    public function setLanguage(string $language): Invoice
     {
         $this->language = $language;
+
+        return $this;
     }
 
-    public function getLanguage(): string
+    public function getLanguage(): ?string
     {
         return $this->language;
     }
 
-    public function getDiscount(): float
+    public function getDiscount(): ?float
     {
         return $this->discount;
     }
@@ -282,7 +321,7 @@ public function setDiscountIsPercentage(bool $discountIsPercentage): Invoice
         return $this;
     }
 
-    public function getDiscountDescription(): string
+    public function getDiscountDescription(): ?string
     {
         return $this->discountDescription;
     }
@@ -299,7 +338,7 @@ public function prepareForSending(): void
         $this->prepareForSending = true;
     }
 
-    public function getVatException(): string
+    public function getVatException(): ?string
     {
         return $this->vatException;
     }
@@ -311,7 +350,7 @@ public function setVatException(string $vatException): Invoice
         return $this;
     }
 
-    public function getVatDescription(): string
+    public function getVatDescription(): ?string
     {
         return $this->vatDescription;
     }
@@ -335,7 +374,7 @@ public static function initializeWithRawData(array $data): Invoice
         if(isset($data['id'])) $item->setId($data['id']);
         if(isset($data['client_id'])) $item->setClientId($data['client_id']);
         if(isset($data['iid'])) $item->setIid($data['iid']);
-        if(isset($data['state'])) $item->setState($data['state']);
+        if(isset($data['state'])) $item->setState(new State($data['state']));
         if(isset($data['payment_method'])) $item->setPaymentMethod($data['payment_method']);
         if(isset($data['generated'])) $item->setGenerated(new DateTime('@' . strtotime($data['generated'])));
         if(isset($data['description'])) $item->setDescription($data['description']);
@@ -375,7 +414,7 @@ public function toArray(bool $forApi = false): array
     {
         $data = array();
         $data['client_id'] = $this->getClientId();
-        $data['state'] = $this->getState();
+        $data['state'] = $this->getState()->getValue();
         $data['payment_method'] = $this->getPaymentMethod();
         $data['description'] = $this->getDescription();
         $data['shown_remark'] = $this->getShownRemark();
@@ -413,7 +452,7 @@ public function toArray(bool $forApi = false): array
             foreach ($this->getHistory() as $history) {
                 $data['history'][] = $history->toArray($forApi);
             }
-        } else {
+        } elseif ($this->prepareForSending) {
             $data['prepare_for_sending'] = $this->prepareForSending;
         }
 
diff --git a/src/Invoice/Item.php b/src/Invoice/Item.php
index e00130f..0467162 100644
--- a/src/Invoice/Item.php
+++ b/src/Invoice/Item.php
@@ -11,35 +11,59 @@ class Item
 {
     protected string $description;
 
-    protected string $externalProductId;
-
     protected float $amount;
 
     protected float $price;
 
-    protected float $totalWithoutVat;
+    protected ?int $vat;
+
+    protected ?string $externalProductId;
 
-    protected float $totalVat;
+    protected ?float $totalWithoutVat;
 
-    protected float $totalWithVat;
+    protected ?float $totalVat;
 
-    protected float $totalVatOverrule;
+    protected ?float $totalWithVat;
 
-    protected ?int $vat;
+    protected ?float $totalVatOverrule;
 
-    protected int $referenceId;
+    protected ?int $referenceId;
 
-    protected float $discount;
+    protected ?float $discount;
 
     protected bool $discountIsPercentage = false;
 
-    protected string $discountDescription;
+    protected ?string $discountDescription;
 
     protected ?int $productId;
 
-    public function setAmount(float $amount): void
+    public function __construct(
+        string $description,
+        float $amount,
+        float $price,
+        ?int $vat = null
+    ) {
+        $this->description = $description;
+        $this->amount = $amount;
+        $this->price = $price;
+        $this->vat = $vat;
+
+        $this->externalProductId = null;
+        $this->totalWithoutVat = null;
+        $this->totalVat = null;
+        $this->totalWithVat = null;
+        $this->totalVatOverrule = null;
+        $this->referenceId = null;
+        $this->discount = null;
+        $this->discountDescription = null;
+        $this->productId = null;
+    }
+
+    public function setAmount(float $amount): Item
     {
         $this->amount = $amount;
+
+        return $this;
     }
 
     public function getAmount(): float
@@ -47,9 +71,11 @@ public function getAmount(): float
         return $this->amount;
     }
 
-    public function setDescription(string $description): void
+    public function setDescription(string $description): Item
     {
         $this->description = $description;
+
+        return $this;
     }
 
     public function getDescription(): string
@@ -57,9 +83,11 @@ public function getDescription(): string
         return $this->description;
     }
 
-    public function setPrice(float $price): void
+    public function setPrice(float $price): Item
     {
         $this->price = $price;
+
+        return $this;
     }
 
     public function getPrice(): float
@@ -72,7 +100,7 @@ private function setReferenceId(int $referenceId): void
         $this->referenceId = $referenceId;
     }
 
-    public function getReferenceId(): int
+    public function getReferenceId(): ?int
     {
         return $this->referenceId;
     }
@@ -82,7 +110,7 @@ private function setTotalVat(float $totalVat): void
         $this->totalVat = $totalVat;
     }
 
-    public function getTotalVat(): float
+    public function getTotalVat(): ?float
     {
         return $this->totalVat;
     }
@@ -92,7 +120,7 @@ private function setTotalWithVat(float $totalWithVat): void
         $this->totalWithVat = $totalWithVat;
     }
 
-    public function getTotalWithVat(): float
+    public function getTotalWithVat(): ?float
     {
         return $this->totalWithVat;
     }
@@ -102,14 +130,16 @@ private function setTotalWithoutVat(float $totalWithoutVat): void
         $this->totalWithoutVat = $totalWithoutVat;
     }
 
-    public function getTotalWithoutVat(): float
+    public function getTotalWithoutVat(): ?float
     {
         return $this->totalWithoutVat;
     }
 
-    public function setVat(?int $vat = null): void
+    public function setVat(?int $vat = null): Item
     {
         $this->vat = $vat;
+
+        return $this;
     }
 
     public function getVat(): ?int
@@ -117,17 +147,19 @@ public function getVat(): ?int
         return $this->vat;
     }
 
-    public function setTotalVatOverrule(float $vatAmount): void
+    public function setTotalVatOverrule(float $vatAmount): Item
     {
         $this->totalVatOverrule = $vatAmount;
+
+        return $this;
     }
 
-    public function getTotalVatOverrule(): float
+    public function getTotalVatOverrule(): ?float
     {
         return $this->totalVatOverrule;
     }
 
-    public function getDiscount(): float
+    public function getDiscount(): ?float
     {
         return $this->discount;
     }
@@ -151,7 +183,7 @@ public function setDiscountIsPercentage(bool $discountIsPercentage): Item
         return $this;
     }
 
-    public function getDiscountDescription(): string
+    public function getDiscountDescription(): ?string
     {
         return $this->discountDescription;
     }
@@ -163,19 +195,23 @@ public function setDiscountDescription(string $discountDescription): Item
         return $this;
     }
 
-    public function getExternalProductId(): string
+    public function getExternalProductId(): ?string
     {
         return $this->externalProductId;
     }
 
-    public function setExternalProductId(string $externalProductId): void
+    public function setExternalProductId(string $externalProductId): Item
     {
         $this->externalProductId = $externalProductId;
+
+        return $this;
     }
 
-    public function setProductId(?int $productId): void
+    public function setProductId(?int $productId): Item
     {
         $this->productId = $productId;
+
+        return $this;
     }
 
     public function getProductId(): ?int
@@ -188,7 +224,12 @@ public function getProductId(): ?int
      */
     public static function initializeWithRawData(array $data): Item
     {
-        $item = new Item();
+        $item = new Item(
+            $data['description'],
+            $data['amount'],
+            $data['price'],
+            $data['vat'] ?? null
+        );
 
         if(isset($data['description'])) $item->setDescription($data['description']);
         if(isset($data['amount'])) $item->setAmount($data['amount']);
diff --git a/src/ValueObject/State.php b/src/ValueObject/State.php
new file mode 100644
index 0000000..5da8454
--- /dev/null
+++ b/src/ValueObject/State.php
@@ -0,0 +1,80 @@
+<?php
+
+namespace SumoCoders\DeFactuur\ValueObject;
+
+use InvalidArgumentException;
+
+final class State
+{
+    private const PAID = 'paid';
+    private const SENT = 'sent';
+    private const CREATED = 'created';
+    private const GENERATED = 'generated';
+    private const JURIDICIAL_PROCEEDING = 'juridicial_proceedings';
+    private const IRRECOVERABLE = 'irrecoverable';
+
+    private const ALLOWED_VALUES = [
+        self::PAID,
+        self::SENT,
+        self::CREATED,
+        self::GENERATED,
+        self::JURIDICIAL_PROCEEDING,
+        self::IRRECOVERABLE,
+    ];
+
+    private string $value;
+
+    public function __construct(string $value)
+    {
+        $this->value = $value;
+
+        $this->validate();
+    }
+
+    private function validate(): void
+    {
+        if (!in_array($this->value, self::ALLOWED_VALUES)) {
+            throw new InvalidArgumentException($this->value);
+        }
+    }
+
+    public function getValue(): string
+    {
+        return $this->value;
+    }
+
+    public function __toString(): string
+    {
+        return $this->value;
+    }
+
+    public static function paid(): State
+    {
+        return new self(self::PAID);
+    }
+
+    public static function created(): State
+    {
+        return new self(self::CREATED);
+    }
+
+    public static function sent(): State
+    {
+        return new self(self::SENT);
+    }
+
+    public static function generated(): State
+    {
+        return new self(self::GENERATED);
+    }
+
+    public function juridicialProceedings(): State
+    {
+        return new self(self::JURIDICIAL_PROCEEDING);
+    }
+
+    public function irrecoverable(): State
+    {
+        return new self(self::IRRECOVERABLE);
+    }
+}