diff --git a/src/Routing/ResponsePrep/ContentLengthPrep.php b/src/Routing/ResponsePrep/ContentLengthPrep.php
new file mode 100644
index 0000000..863fd7a
--- /dev/null
+++ b/src/Routing/ResponsePrep/ContentLengthPrep.php
@@ -0,0 +1,31 @@
+<?php
+
+namespace WellRESTed\Routing\ResponsePrep;
+
+use Psr\Http\Message\ResponseInterface;
+use Psr\Http\Message\ServerRequestInterface;
+use WellRESTed\Routing\MiddlewareInterface;
+
+/**
+ * Adds a Content-length header to the response when all of these are true:
+ *
+ * - Response does not have a Content-length header
+ * - Response does not have a Tranfser-encoding: chunked header
+ * - Response body stream reports a size
+ */
+class ContentLengthPrep implements MiddlewareInterface
+{
+    public function dispatch(ServerRequestInterface $request, ResponseInterface &$response)
+    {
+        if ($response->hasHeader("Content-length")) {
+            return;
+        }
+        if (strtolower($response->getHeaderLine("Transfer-encoding")) === "chunked") {
+            return;
+        }
+        $size = $response->getBody()->getSize();
+        if ($size !== null) {
+            $response = $response->withHeader("Content-length", (string) $size);
+        }
+    }
+}
diff --git a/test/tests/unit/Routing/ResponsePrep/ContentLengthPrepTest.php b/test/tests/unit/Routing/ResponsePrep/ContentLengthPrepTest.php
new file mode 100644
index 0000000..91a46c8
--- /dev/null
+++ b/test/tests/unit/Routing/ResponsePrep/ContentLengthPrepTest.php
@@ -0,0 +1,80 @@
+<?php
+
+namespace WellRESTed\Test\Unit\Routing;
+
+use Prophecy\Argument;
+use WellRESTed\Routing\ResponsePrep\ContentLengthPrep;
+
+/**
+ * @covers WellRESTed\Routing\ResponsePrep\ContentLengthPrep
+ */
+class ContentLengthPrepTest extends \PHPUnit_Framework_TestCase
+{
+    private $request;
+    private $response;
+    private $body;
+
+    public function setUp()
+    {
+        parent::setUp();
+        $this->body = $this->prophesize('Psr\Http\Message\StreamInterface');
+        $this->body->getSize()->willReturn(1024);
+        $this->request = $this->prophesize('Psr\Http\Message\ServerRequestInterface');
+        $this->response = $this->prophesize('Psr\Http\Message\ResponseInterface');
+        $this->response->getBody()->willReturn($this->body->reveal());
+        $this->response->withHeader(Argument::cetera())->willReturn($this->response->reveal());
+    }
+
+    public function testAddContentLengthHeader()
+    {
+        $this->response->hasHeader("Content-length")->willReturn(false);
+        $this->response->getHeaderLine("Transfer-encoding")->willReturn("");
+
+        $request = $this->request->reveal();
+        $response = $this->response->reveal();
+        $prep = new ContentLengthPrep();
+        $prep->dispatch($request, $response);
+
+        $this->response->withHeader("Content-length", 1024)->shouldHaveBeenCalled();
+    }
+
+    public function testDoesNotAddHeaderWhenContentLenghtIsAlreadySet()
+    {
+        $this->response->hasHeader("Content-length")->willReturn(true);
+        $this->response->getHeaderLine("Transfer-encoding")->willReturn("");
+
+        $request = $this->request->reveal();
+        $response = $this->response->reveal();
+        $prep = new ContentLengthPrep();
+        $prep->dispatch($request, $response);
+
+        $this->response->withHeader(Argument::cetera())->shouldNotHaveBeenCalled();
+    }
+
+    public function testDoesNotAddHeaderWhenTransferEncodingIsChunked()
+    {
+        $this->response->hasHeader("Content-length")->willReturn(false);
+        $this->response->getHeaderLine("Transfer-encoding")->willReturn("CHUNKED");
+
+        $request = $this->request->reveal();
+        $response = $this->response->reveal();
+        $prep = new ContentLengthPrep();
+        $prep->dispatch($request, $response);
+
+        $this->response->withHeader(Argument::cetera())->shouldNotHaveBeenCalled();
+    }
+
+    public function testDoesNotAddHeaderWhenBodySizeIsNull()
+    {
+        $this->response->hasHeader("Content-length")->willReturn(false);
+        $this->response->getHeaderLine("Transfer-encoding")->willReturn("");
+        $this->body->getSize()->willReturn(null);
+
+        $request = $this->request->reveal();
+        $response = $this->response->reveal();
+        $prep = new ContentLengthPrep();
+        $prep->dispatch($request, $response);
+
+        $this->response->withHeader(Argument::cetera())->shouldNotHaveBeenCalled();
+    }
+}