diff --git a/dune-project b/dune-project
index 6af5f58..5b18faf 100644
--- a/dune-project
+++ b/dune-project
@@ -21,7 +21,7 @@
  (name pgx)
  (synopsis "Pure-OCaml PostgreSQL client library")
  (description
-   "PGX is a pure-OCaml PostgreSQL client library, supporting Async, LWT, or synchronous operations.")
+  "PGX is a pure-OCaml PostgreSQL client library, supporting Async, LWT, or synchronous operations.")
  (depends
   (alcotest
    (and
@@ -52,9 +52,9 @@
 (package
  (name pgx_unix)
  (synopsis
-   "PGX using the standard library's Unix module for IO (synchronous)")
+  "PGX using the standard library's Unix module for IO (synchronous)")
  (description
-   "PGX using the standard library's Unix module for IO (synchronous)")
+  "PGX using the standard library's Unix module for IO (synchronous)")
  (depends
   (alcotest
    (and
@@ -82,10 +82,12 @@
    (>= "v0.13.0"))
   (async_unix
    (>= "v0.13.0"))
+  async_ssl
   (base64
    (and
     :with-test
     (>= 3.0.0)))
+  conduit-async
   (ocaml
    (>= 4.08))
   (pgx
diff --git a/pgx/src/io_intf.ml b/pgx/src/io_intf.ml
index 867ac38..00babdd 100644
--- a/pgx/src/io_intf.ml
+++ b/pgx/src/io_intf.ml
@@ -14,6 +14,18 @@ module type S = sig
     | Inet of string * int
 
   val open_connection : sockaddr -> (in_channel * out_channel) t
+
+  type ssl_config
+
+  val upgrade_ssl
+    : [ `Not_supported
+      | `Supported of
+        ?ssl_config:ssl_config
+        -> in_channel
+        -> out_channel
+        -> (in_channel * out_channel) t
+      ]
+
   val output_char : out_channel -> char -> unit t
   val output_binary_int : out_channel -> int -> unit t
   val output_string : out_channel -> string -> unit t
diff --git a/pgx/src/pgx.ml b/pgx/src/pgx.ml
index 9c00400..4ae77e5 100644
--- a/pgx/src/pgx.ml
+++ b/pgx/src/pgx.ml
@@ -288,6 +288,7 @@ module Message_out = struct
     | Describe_portal of portal (* DP *)
     | Startup_message of startup
     | Simple_query of query
+    | SSLRequest
   [@@deriving sexp]
 
   let add_byte buf i =
@@ -381,6 +382,10 @@ module Message_out = struct
       add_byte msg 0;
       None, Buffer.contents msg
     | Simple_query q -> Some 'Q', str q
+    | SSLRequest ->
+      let msg = Buffer.create 8 in
+      add_int32 msg 80877103l;
+      None, Buffer.contents msg
   ;;
 end
 
@@ -526,7 +531,59 @@ module Make (Thread : Io) = struct
 
   (*----- Connection. -----*)
 
+  let attempt_tls_upgrade ?(ssl = `Auto) ({ ichan; chan; _ } as conn) =
+    (* To initiate an SSL-encrypted connection, the frontend initially sends an SSLRequest message rather than a
+       StartupMessage. The server then responds with a single byte containing S or N, indicating that it is willing
+       or unwilling to perform SSL, respectively. The frontend might close the connection at this point if it is
+        dissatisfied with the response. To continue after S, perform an SSL startup handshake (not described here,
+       part of the SSL specification) with the server. If this is successful, continue with sending the usual
+       StartupMessage. In this case the StartupMessage and all subsequent data will be SSL-encrypted. To continue
+       after N, send the usual StartupMessage and proceed without encryption.
+       See https://www.postgresql.org/docs/9.3/protocol-flow.html#AEN100021 *)
+    match ssl with
+    | `No -> return conn
+    | (`Auto | `Always _) as ssl ->
+      (match Io.upgrade_ssl with
+      | `Not_supported ->
+        (match ssl with
+        | `Always _ ->
+          failwith
+            "TLS support is not compiled into this Pgx library but ~ssl was set to \
+             `Always"
+        | _ -> ());
+        debug
+          "TLS-support is not compiled into this Pgx library, not attempting to upgrade"
+        >>| fun () -> conn
+      | `Supported upgrade_ssl ->
+        debug "Request SSL upgrade from server"
+        >>= fun () ->
+        let msg = Message_out.SSLRequest in
+        send_message conn msg
+        >>= fun () ->
+        flush chan
+        >>= fun () ->
+        input_char ichan
+        >>= (function
+        | 'S' ->
+          debug "Server supports TLS, attempting to upgrade"
+          >>= fun () ->
+          let ssl_config =
+            match ssl with
+            | `Auto -> None
+            | `Always ssl_config -> Some ssl_config
+          in
+          upgrade_ssl ?ssl_config ichan chan
+          >>= fun (ichan, chan) -> return { conn with ichan; chan }
+        | 'N' -> debug "Server does not support TLS, not upgrading" >>| fun () -> conn
+        | c ->
+          fail_msg
+            "Got unexpected response '%c' from server after SSLRequest message. Response \
+             should always be 'S' or 'N'."
+            c))
+  ;;
+
   let connect
+      ?ssl
       ?host
       ?port
       ?user
@@ -600,6 +657,8 @@ module Make (Thread : Io) = struct
       ; prepared_num = Int64.of_int 0
       }
     in
+    attempt_tls_upgrade ?ssl conn
+    >>= fun conn ->
     (* Send the StartUpMessage.  NB. At present we do not support SSL. *)
     let msg = Message_out.Startup_message { Message_out.user; database } in
     (* Loop around here until the database gives a ReadyForQuery message. *)
@@ -665,6 +724,7 @@ module Make (Thread : Io) = struct
   ;;
 
   let with_conn
+      ?ssl
       ?host
       ?port
       ?user
@@ -676,6 +736,7 @@ module Make (Thread : Io) = struct
       f
     =
     connect
+      ?ssl
       ?host
       ?port
       ?user
diff --git a/pgx/src/pgx.mli b/pgx/src/pgx.mli
index 526f400..cbf5ec6 100644
--- a/pgx/src/pgx.mli
+++ b/pgx/src/pgx.mli
@@ -49,4 +49,5 @@ module Value = Pgx_value
 
 module type S = Pgx_intf.S
 
-module Make (Thread : Io) : S with type 'a Io.t = 'a Thread.t
+module Make (Thread : Io) :
+  S with type 'a Io.t = 'a Thread.t and type Io.ssl_config = Thread.ssl_config
diff --git a/pgx/src/pgx_intf.ml b/pgx/src/pgx_intf.ml
index f6167ad..6bd16e4 100644
--- a/pgx/src/pgx_intf.ml
+++ b/pgx/src/pgx_intf.ml
@@ -5,6 +5,7 @@ module type S = sig
 
   module Io : sig
     type 'a t
+    type ssl_config
 
     val return : 'a -> 'a t
     val ( >>= ) : 'a t -> ('a -> 'b t) -> 'b t
@@ -22,7 +23,8 @@ module type S = sig
       possible denial of service.  You may want to set this to a smaller
       size to avoid this happening. *)
   val connect
-    :  ?host:string
+    :  ?ssl:[ `Auto | `No | `Always of Io.ssl_config ]
+    -> ?host:string
     -> ?port:int
     -> ?user:string
     -> ?password:string
@@ -42,7 +44,8 @@ module type S = sig
       [close]. This is the preferred way to use this library since it cleans up
       after itself. *)
   val with_conn
-    :  ?host:string
+    :  ?ssl:[ `Auto | `No | `Always of Io.ssl_config ]
+    -> ?host:string
     -> ?port:int
     -> ?user:string
     -> ?password:string
diff --git a/pgx_async.opam b/pgx_async.opam
index e9dbcfe..b6a7eff 100644
--- a/pgx_async.opam
+++ b/pgx_async.opam
@@ -13,7 +13,9 @@ depends: [
   "alcotest-async" {with-test & >= "1.0.0"}
   "async_kernel" {>= "v0.13.0"}
   "async_unix" {>= "v0.13.0"}
+  "async_ssl"
   "base64" {with-test & >= "3.0.0"}
+  "conduit-async"
   "ocaml" {>= "4.08"}
   "pgx" {= version}
   "pgx_value_core" {= version}
diff --git a/pgx_async/src/dune b/pgx_async/src/dune
index 5e63847..e361930 100644
--- a/pgx_async/src/dune
+++ b/pgx_async/src/dune
@@ -11,6 +11,6 @@ let () = Jbuild_plugin.V1.send @@ {|
 (library
  (public_name pgx_async)
  (wrapped false)
- (libraries async_kernel async_unix pgx_value_core)
+ (libraries async_kernel async_unix conduit-async pgx_value_core)
  |} ^ preprocess ^ {|)
 |}
diff --git a/pgx_async/src/pgx_async.ml b/pgx_async/src/pgx_async.ml
index 7aec8ae..0e3ac26 100644
--- a/pgx_async/src/pgx_async.ml
+++ b/pgx_async/src/pgx_async.ml
@@ -73,19 +73,25 @@ module Thread = struct
   let close_in = Reader.close
 
   let open_connection sockaddr =
-    let get_reader_writer socket =
-      let fd = Socket.fd socket in
-      Reader.create fd, Writer.create fd
-    in
     match sockaddr with
-    | Unix path ->
-      let unix_sockaddr = Tcp.Where_to_connect.of_unix_address (`Unix path) in
-      Tcp.connect_sock unix_sockaddr >>| get_reader_writer
+    | Unix path -> Conduit_async.connect (`Unix_domain_socket path)
     | Inet (host, port) ->
-      let inet_sockaddr =
-        Tcp.Where_to_connect.of_host_and_port (Host_and_port.create ~host ~port)
-      in
-      Tcp.connect_sock inet_sockaddr >>| get_reader_writer
+      Uri.make ~host ~port ()
+      |> Conduit_async.V3.resolve_uri
+      >>= Conduit_async.V3.connect
+      >>| fun (_socket, in_channel, out_channel) -> in_channel, out_channel
+  ;;
+
+  type ssl_config = Conduit_async.Ssl.config
+
+  let upgrade_ssl =
+    try
+      let default_config = Conduit_async.V1.Conduit_async_ssl.Ssl_config.configure () in
+      `Supported
+        (fun ?(ssl_config = default_config) in_channel out_channel ->
+          Conduit_async.V1.Conduit_async_ssl.ssl_connect ssl_config in_channel out_channel)
+    with
+    | _ -> `Not_supported
   ;;
 
   (* The unix getlogin syscall can fail *)
@@ -130,6 +136,7 @@ let check_pgdatabase =
 ;;
 
 let connect
+    ?ssl
     ?host
     ?port
     ?user
@@ -146,6 +153,7 @@ let connect
   | None -> Lazy_deferred.force_exn default_unix_domain_socket_dir)
   >>= fun unix_domain_socket_dir ->
   connect
+    ?ssl
     ?host
     ?port
     ?user
@@ -158,6 +166,7 @@ let connect
 ;;
 
 let with_conn
+    ?ssl
     ?host
     ?port
     ?user
@@ -169,6 +178,7 @@ let with_conn
     f
   =
   connect
+    ?ssl
     ?host
     ?port
     ?user
diff --git a/pgx_async/src/pgx_async.mli b/pgx_async/src/pgx_async.mli
index 8adc38f..f816033 100644
--- a/pgx_async/src/pgx_async.mli
+++ b/pgx_async/src/pgx_async.mli
@@ -1,23 +1,14 @@
 (** Async based Postgres client based on Pgx. *)
 open Async_kernel
 
-include Pgx.S with type 'a Io.t = 'a Deferred.t
+include
+  Pgx.S
+    with type 'a Io.t = 'a Deferred.t
+     and type Io.ssl_config = Conduit_async.Ssl.config
 
 (* for testing purposes *)
 module Thread : Pgx.Io with type 'a t = 'a Deferred.t
 
-val with_conn
-  :  ?host:string
-  -> ?port:int
-  -> ?user:string
-  -> ?password:string
-  -> ?database:string
-  -> ?unix_domain_socket_dir:string
-  -> ?verbose:int
-  -> ?max_message_length:int
-  -> (t -> 'a Deferred.t)
-  -> 'a Deferred.t
-
 (** Like [execute] but returns a pipe so you can operate on the results before they have all returned.
     Note that [execute_iter] and [execute_fold] can perform significantly better because they don't have
     as much overhead. *)
diff --git a/pgx_lwt/src/pgx_lwt.ml b/pgx_lwt/src/pgx_lwt.ml
index 70f827d..c70ac8a 100644
--- a/pgx_lwt/src/pgx_lwt.ml
+++ b/pgx_lwt/src/pgx_lwt.ml
@@ -47,6 +47,8 @@ module Thread = struct
 
     let close_in = Io.close_in
     let open_connection = Io.open_connection
+    type ssl_config
+    let upgrade_ssl = `Not_supported
     let getlogin = Io.getlogin
     let debug s = Logs_lwt.debug (fun m -> m "%s" s)
     let protect f ~finally = Lwt.finalize f finally
diff --git a/pgx_lwt_unix.opam b/pgx_lwt_unix.opam
index 9a8e7e0..3163469 100644
--- a/pgx_lwt_unix.opam
+++ b/pgx_lwt_unix.opam
@@ -12,7 +12,6 @@ depends: [
   "dune" {>= "1.11"}
   "alcotest-lwt" {with-test & >= "1.0.0"}
   "base64" {with-test & >= "3.0.0"}
-  "lwt"
   "ocaml" {>= "4.08"}
   "pgx" {= version}
   "pgx_lwt" {= version}
diff --git a/pgx_unix/src/pgx_unix.ml b/pgx_unix/src/pgx_unix.ml
index d0b94b6..965f7f6 100644
--- a/pgx_unix/src/pgx_unix.ml
+++ b/pgx_unix/src/pgx_unix.ml
@@ -55,6 +55,9 @@ module Simple_thread = struct
     Unix.open_connection std_socket
   ;;
 
+  type ssl_config
+
+  let upgrade_ssl = `Not_supported
   let output_char = output_char
   let output_binary_int = output_binary_int
   let output_string = output_string