diff --git a/README.markdown b/README.markdown
index c7ea07f29b..3d8ce3baf1 100644
--- a/README.markdown
+++ b/README.markdown
@@ -192,6 +192,24 @@ post '/json_endpoint' do
 end
 ```
 
+## Validations
+
+You can define validations and coercion option for your attributes:
+
+```ruby
+params do
+  required :id, type: Integer
+  optional :name, type: String, regexp: /^[a-z]+$/
+end
+
+get ':id' do
+  # params[:id] is an Integer
+end
+```
+
+When a type is specified an implicit validation is done after the coercion to ensure the output type is what you asked.
+
+
 ## Headers
 
 Headers are available through the `env` hash object.
diff --git a/grape.gemspec b/grape.gemspec
index 133f7cdba2..e5ea244feb 100644
--- a/grape.gemspec
+++ b/grape.gemspec
@@ -20,6 +20,7 @@ Gem::Specification.new do |s|
   s.add_runtime_dependency 'multi_json'
   s.add_runtime_dependency 'multi_xml'
   s.add_runtime_dependency 'hashie', '~> 1.2'
+  s.add_runtime_dependency 'virtus'
 
   s.add_development_dependency 'rake'
   s.add_development_dependency 'maruku'
diff --git a/lib/grape.rb b/lib/grape.rb
index 44d5bf8303..3b8e672bb9 100644
--- a/lib/grape.rb
+++ b/lib/grape.rb
@@ -9,6 +9,7 @@ module Grape
   autoload :Route,           'grape/route'
   autoload :Entity,          'grape/entity'
   autoload :Cookies,         'grape/cookies'
+  autoload :Validations,     'grape/validations'
 
   module Middleware
     autoload :Base,      'grape/middleware/base'
diff --git a/lib/grape/api.rb b/lib/grape/api.rb
index eec25f8e88..d4c46a0612 100644
--- a/lib/grape/api.rb
+++ b/lib/grape/api.rb
@@ -8,6 +8,8 @@ module Grape
   # creating Grape APIs.Users should subclass this
   # class in order to build an API.
   class API
+    extend Validations::ClassMethods
+    
     class << self
       attr_reader :route_set
       attr_reader :versions
@@ -32,6 +34,7 @@ def reset!
         @endpoints = []
         @mountings = []
         @routes = nil
+        reset_validations!
       end
 
       def compile
@@ -287,7 +290,9 @@ def route(methods, paths = ['/'], route_options = {}, &block)
           :route_options => (route_options || {}).merge(@last_description || {})
         }
         endpoints << Grape::Endpoint.new(settings.clone, endpoint_options, &block)
+        
         @last_description = nil
+        reset_validations!
       end
 
       def before(&block)
diff --git a/lib/grape/endpoint.rb b/lib/grape/endpoint.rb
index 3ec4ad3eb0..d500fa18d2 100644
--- a/lib/grape/endpoint.rb
+++ b/lib/grape/endpoint.rb
@@ -285,6 +285,11 @@ def run(env)
 
       self.extend helpers
       cookies.read(@request)
+            
+      Array(settings[:validations]).each do |validator|
+        validator.validate!(params)
+      end
+      
       run_filters befores
       response_text = instance_eval &self.block
       run_filters afters
diff --git a/lib/grape/validations.rb b/lib/grape/validations.rb
new file mode 100644
index 0000000000..fa7a9991dc
--- /dev/null
+++ b/lib/grape/validations.rb
@@ -0,0 +1,154 @@
+require 'virtus'
+
+module Grape
+    
+  module Validations
+    
+    ##
+    # All validators must inherit from this class.
+    # 
+    class Validator
+      def initialize(attrs, options)
+        @attrs = Array(attrs)
+
+        if options.is_a?(Hash) && !options.empty?
+          raise "unknown options: #{options.keys}"
+        end
+      end
+
+      def validate!(params)
+        @attrs.each do |attr_name|
+          validate_param!(attr_name, params)
+        end
+      end
+
+    private
+
+      def self.convert_to_short_name(klass)
+        ret = klass.name.gsub(/::/, '/').
+          gsub(/([A-Z]+)([A-Z][a-z])/,'\1_\2').
+          gsub(/([a-z\d])([A-Z])/,'\1_\2').
+          tr("-", "_").
+          downcase
+        File.basename(ret, '_validator')
+      end
+    end
+
+    ##
+    # Base class for all validators taking only one param.
+    class SingleOptionValidator < Validator
+      def initialize(attrs, options)
+        @option = options
+        super
+      end
+
+    end
+
+    # we define Validator::inherited here so SingleOptionValidator
+    # will not be considered a validator.
+    class Validator
+      def self.inherited(klass)
+        short_name = convert_to_short_name(klass)
+        Validations::register_validator(short_name, klass)
+      end
+    end
+    
+    
+    
+    class <<self
+      attr_accessor :validators
+    end
+    
+    self.validators = {}
+    
+    def self.register_validator(short_name, klass)
+      validators[short_name] = klass
+    end
+    
+    
+    class ParamsScope
+      def initialize(api, &block)
+        @api = api
+        instance_eval(&block)
+      end
+      
+      def requires(*attrs)
+        validations = {:presence => true}
+        if attrs.last.is_a?(Hash)
+          validations.merge!(attrs.pop)
+        end
+        
+        validates(attrs, validations)
+      end
+      
+      def optional(*attrs)
+        validations = {}
+        if attrs.last.is_a?(Hash)
+          validations.merge!(attrs.pop)
+        end
+        
+        validates(attrs, validations)
+      end
+      
+    private
+      def validates(attrs, validations)
+        doc_attrs = { :required => validations.keys.include?(:presence) }
+        
+        # special case (type = coerce)
+        if validations[:type]
+          validations[:coerce] = validations.delete(:type)
+        end
+        
+        if coerce_type = validations[:coerce]
+          doc_attrs[:type] = coerce_type.to_s
+        end
+        
+        if desc = validations.delete(:desc)
+          doc_attrs[:desc] = desc
+        end
+        
+        @api.document_attribute(attrs, doc_attrs)
+        
+        validations.each do |type, options|
+          validator_class = Validations::validators[type.to_s]
+          if validator_class
+            @api.settings[:validations] << validator_class.new(attrs, options)
+          else
+            raise "unknown validator: #{type}"
+          end
+        end
+      
+      end
+      
+    end
+    
+    # This module is mixed into the API Class.
+    module ClassMethods
+      def reset_validations!
+        settings[:validations] = []
+      end
+      
+      def params(&block)
+        ParamsScope.new(self, &block)
+      end
+      
+      def document_attribute(names, opts)
+        if @last_description
+          @last_description[:params] ||= {}
+        
+          Array(names).each do |name|
+            @last_description[:params][name.to_sym] ||= {}
+            @last_description[:params][name.to_sym].merge!(opts)
+          end
+        end
+      end
+      
+    end
+    
+  end
+end
+
+# load all defined validations
+Dir[File.expand_path('../validations/*.rb', __FILE__)].each do |path|
+  require(path)
+end
diff --git a/lib/grape/validations/coerce.rb b/lib/grape/validations/coerce.rb
new file mode 100644
index 0000000000..865a4f045f
--- /dev/null
+++ b/lib/grape/validations/coerce.rb
@@ -0,0 +1,56 @@
+
+module Grape
+  class API
+    Boolean = Virtus::Attribute::Boolean
+  end
+  
+  module Validations
+    
+    class CoerceValidator < SingleOptionValidator
+      def validate_param!(attr_name, params)
+        new_value = coerce_value(@option, params[attr_name])
+        if valid_type?(new_value)
+          params[attr_name] = new_value
+        else
+          throw :error, :status => 400, :message => "invalid parameter: #{attr_name}"
+        end
+      end
+  
+    private
+      def _valid_array_type?(type, values)
+        values.all? do |val|
+          _valid_single_type?(type, val)
+        end
+      end
+      
+      
+      def _valid_single_type?(klass, val)
+        if klass == Virtus::Attribute::Boolean
+          val.is_a?(TrueClass) || val.is_a?(FalseClass)
+        else
+          val.is_a?(klass)
+        end
+      end
+      
+      def valid_type?(val)
+        if @option.is_a?(Array)
+          _valid_array_type?(@option[0], val)
+        else
+          _valid_single_type?(@option, val)
+        end
+      end
+      
+      def coerce_value(type, val)
+        converter = Virtus::Attribute.build(:a, type)
+        converter.coerce(val)
+      
+      # not the prettiest but some invalid coercion can currently trigger
+      # errors in Virtus (see coerce_spec.rb)
+      rescue => err
+        nil
+      end
+      
+    end
+    
+  end
+end
diff --git a/lib/grape/validations/presence.rb b/lib/grape/validations/presence.rb
new file mode 100644
index 0000000000..a6871e2086
--- /dev/null
+++ b/lib/grape/validations/presence.rb
@@ -0,0 +1,14 @@
+module Grape
+  module Validations
+    
+    class PresenceValidator < Validator
+      def validate_param!(attr_name, params)
+        unless params.has_key?(attr_name)
+          throw :error, :status => 400, :message => "missing parameter: #{attr_name}"
+        end
+      end
+    
+    end
+    
+  end
+end
diff --git a/lib/grape/validations/regexp.rb b/lib/grape/validations/regexp.rb
new file mode 100644
index 0000000000..9b1081730c
--- /dev/null
+++ b/lib/grape/validations/regexp.rb
@@ -0,0 +1,13 @@
+module Grape
+  module Validations
+    
+    class RegexpValidator < SingleOptionValidator
+      def validate_param!(attr_name, params)
+        if params[attr_name] && !( params[attr_name].to_s =~ @option )
+          throw :error, :status => 400, :message => "invalid parameter: #{attr_name}"
+        end
+      end
+    end
+
+  end
+end
diff --git a/spec/grape/validations/coerce_spec.rb b/spec/grape/validations/coerce_spec.rb
new file mode 100644
index 0000000000..087f214efa
--- /dev/null
+++ b/spec/grape/validations/coerce_spec.rb
@@ -0,0 +1,95 @@
+require 'spec_helper'
+
+require 'virtus'
+
+module CoerceTest
+  class User
+    include Virtus
+    
+    attribute :id, Integer
+    attribute :name, String
+  end
+  
+  class API < Grape::API
+    default_format :json
+
+
+    params do
+      requires :int, :coerce => Integer
+    end
+    get '/single' do
+    end
+
+    params do
+      requires :ids, type: Array[Integer]
+    end
+    get '/arr' do
+    end
+
+    params do
+      requires :user, type: CoerceTest::User
+    end
+    get '/user' do
+    end
+
+    params do
+      requires :int, :coerce => Integer
+      optional :arr, :coerce => Array[Integer]
+      optional :bool, :coerce => Array[Boolean]
+    end
+    get '/coerce' do
+      {
+        :int    => params[:int].class,
+        :arr    => params[:arr] ? params[:arr][0].class : nil,
+        :bool   => params[:bool] ? (params[:bool][0] == true) && (params[:bool][1] == false) : nil
+      }
+    end
+  end
+end
+
+describe Grape::Validations::CoerceValidator do
+  def app
+    CoerceTest::API
+  end
+    
+  it "should return an error on malformed input" do
+    get '/single', :int => "43a"
+    last_response.status.should == 400
+    
+    get '/single', :int => "43"
+    last_response.status.should == 200
+  end
+  
+  it "should return an error on malformed input (array)" do
+    get '/arr', :ids => ["1", "2", "az"]
+    last_response.status.should == 400
+    
+    get '/arr', :ids => ["1", "2", "890"]
+    last_response.status.should == 200
+  end
+  
+  it "should return an error on malformed input (complex object)" do
+    # this request does raise an error inside Virtus
+    get '/user', :user => "32"
+    last_response.status.should == 400
+    
+    get '/user', :user => {id: 32, name: "Bob"}
+    last_response.status.should == 200
+  end
+  
+  it 'should coerce inputs' do
+    get('/coerce', :int => "43")
+    last_response.status.should == 200
+    ret = MultiJson.load(last_response.body)
+    ret["int"].should == "Fixnum"
+    
+    get('/coerce', :int => "40", :arr => ["1","20","3"], :bool => [1, 0])
+    # last_response.body.should == ""
+    last_response.status.should == 200
+    ret = MultiJson.load(last_response.body)
+    ret["int"].should == "Fixnum"
+    ret["arr"].should == "Fixnum"
+    ret["bool"].should == true
+  end
+  
+end
diff --git a/spec/grape/validations/presence_spec.rb b/spec/grape/validations/presence_spec.rb
new file mode 100644
index 0000000000..932bd1fcf9
--- /dev/null
+++ b/spec/grape/validations/presence_spec.rb
@@ -0,0 +1,58 @@
+require 'spec_helper'
+
+describe Grape::Validations::PresenceValidator do
+  def app; @app; end
+  
+  before do
+    @app = Class.new(Grape::API) do
+      default_format :json
+      
+      params do
+        requires :id, :regexp => /^[0-9]+$/
+      end
+      
+      post do
+        {:ret => params[:id]}
+      end
+      
+      params do
+        requires :name, :company
+      end
+      
+      get do
+        "Hello"
+      end
+      
+    end
+    
+  end
+  
+  it 'validates id' do
+    post('/')
+    last_response.status.should == 400
+    last_response.body.should == "missing parameter: id"
+    
+    post('/', {}, 'rack.input' => StringIO.new('{"id" : "a56b"}'))
+    last_response.body.should == 'invalid parameter: id'
+    last_response.status.should == 400
+    
+    post('/', {}, 'rack.input' => StringIO.new('{"id" : 56}'))
+    last_response.body.should == '{"ret":56}'
+    last_response.status.should == 201
+  end
+  
+  it 'validates name, company' do
+    get('/')
+    last_response.status.should == 400
+    last_response.body.should == "missing parameter: name"
+    
+    get('/', :name => "Bob")
+    last_response.status.should == 400
+    last_response.body.should == "missing parameter: company"
+    
+    get('/', :name => "Bob", :company => "TestCorp")
+    last_response.status.should == 200
+    last_response.body.should == "Hello"
+  end
+  
+end
diff --git a/spec/grape/validations/regexp_spec.rb b/spec/grape/validations/regexp_spec.rb
new file mode 100644
index 0000000000..9c656dc515
--- /dev/null
+++ b/spec/grape/validations/regexp_spec.rb
@@ -0,0 +1,31 @@
+require 'spec_helper'
+
+describe Grape::Validations::RegexpValidator do
+  def app; @app; end
+  
+  before do
+    @app = Class.new(Grape::API) do
+      default_format :json
+      
+      params do
+        requires :name, :regexp => /^[a-z]+$/
+      end
+      get do
+        
+      end
+      
+    end
+    
+  end
+  
+  it 'should refuse invalid input' do
+    get '/', :name => "invalid name"
+    last_response.status.should == 400
+  end
+  
+  it 'should accept valid input' do
+    get '/', :name => "bob"
+    last_response.status.should == 200
+  end
+  
+end
diff --git a/spec/grape/validations_spec.rb b/spec/grape/validations_spec.rb
new file mode 100644
index 0000000000..fd998875b0
--- /dev/null
+++ b/spec/grape/validations_spec.rb
@@ -0,0 +1,33 @@
+require 'spec_helper'
+
+describe Grape::Validations do
+  def app; @app; end
+  
+  before do
+    @app = Class.new(Grape::API) do
+      default_format :json
+      
+      params do
+        requires :name, :company
+        optional :a_number, :regexp => /^[0-9]+$/
+      end
+      
+      get do
+        "Hello"
+      end
+      
+    end
+    
+  end
+  
+  it 'validates optional parameter if present' do
+    get('/', :name => "Bob", :company => "TestCorp", :a_number => "string")
+    last_response.status.should == 400
+    last_response.body.should == "invalid parameter: a_number"
+    
+    get('/', :name => "Bob", :company => "TestCorp", :a_number => 45)
+    last_response.status.should == 200
+    last_response.body.should == "Hello"
+  end
+  
+end