diff --git a/examples/index.php b/examples/index.php
index 7256554..3cc7546 100644
--- a/examples/index.php
+++ b/examples/index.php
@@ -110,6 +110,48 @@
     );
 });
 
+$app->get('/etag/', function (ServerRequestInterface $request) {
+    $etag = '"_"';
+    if ($request->getHeaderLine('If-None-Match') === $etag) {
+        return new React\Http\Message\Response(
+            304,
+            [
+                'ETag' => $etag
+            ],
+            ''
+        );
+    }
+
+    return new React\Http\Message\Response(
+        200,
+        [
+            'ETag' => $etag
+        ],
+        ''
+    );
+});
+$app->get('/etag/{etag:[a-z]+}', function (ServerRequestInterface $request) {
+    $etag = '"' . $request->getAttribute('etag') . '"';
+    if ($request->getHeaderLine('If-None-Match') === $etag) {
+        return new React\Http\Message\Response(
+            304,
+            [
+                'ETag' => $etag,
+                'Content-Length' => strlen($etag) - 1
+            ],
+            ''
+        );
+    }
+
+    return new React\Http\Message\Response(
+        200,
+        [
+            'ETag' => $etag
+        ],
+        $request->getAttribute('etag') . "\n"
+    );
+});
+
 $app->map(['GET', 'POST'], '/headers', function (ServerRequestInterface $request) {
     // Returns a JSON representation of all request headers passed to this endpoint.
     // Note that this assumes UTF-8 data in request headers and may break for other encodings,
diff --git a/src/SapiHandler.php b/src/SapiHandler.php
index c22caad..e34f86f 100644
--- a/src/SapiHandler.php
+++ b/src/SapiHandler.php
@@ -82,11 +82,17 @@ public function requestFromGlobals(): ServerRequestInterface
      */
     public function sendResponse(ResponseInterface $response): void
     {
-        header($_SERVER['SERVER_PROTOCOL'] . ' ' . $response->getStatusCode() . ' ' . $response->getReasonPhrase());
+        $status = $response->getStatusCode();
+        $body = $response->getBody();
+
+        header($_SERVER['SERVER_PROTOCOL'] . ' ' . $status . ' ' . $response->getReasonPhrase());
 
-        // automatically assign "Content-Length" response header if known and not already present
-        if (!$response->hasHeader('Content-Length') && $response->getBody()->getSize() !== null) {
-            $response = $response->withHeader('Content-Length', (string)$response->getBody()->getSize());
+        if ($status === 204) {
+            // 204 MUST NOT include "Content-Length" response header
+            $response = $response->withoutHeader('Content-Length');
+        } elseif (!$response->hasHeader('Content-Length') && $body->getSize() !== null && ($status !== 304 || $body->getSize() !== 0)) {
+            // automatically assign "Content-Length" response header if known and not already present
+            $response = $response->withHeader('Content-Length', (string) $body->getSize());
         }
 
         // remove default "Content-Type" header set by PHP (default_mimetype)
@@ -105,7 +111,10 @@ public function sendResponse(ResponseInterface $response): void
         }
         ini_set('default_charset', $old);
 
-        $body = $response->getBody();
+        if (($_SERVER['REQUEST_METHOD'] ?? '') === 'HEAD' || $status === 204 || $status === 304) {
+            $body->close();
+            return;
+        }
 
         if ($body instanceof ReadableStreamInterface) {
             // try to disable nginx buffering (http://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_buffering)
diff --git a/tests/SapiHandlerTest.php b/tests/SapiHandlerTest.php
index cf7879b..14b7e60 100644
--- a/tests/SapiHandlerTest.php
+++ b/tests/SapiHandlerTest.php
@@ -170,6 +170,95 @@ public function testSendResponseSendsJsonResponseWithGivenHeadersAndBodyAndAssig
         $this->assertEquals(array_merge($previous, ['Content-Type: application/json', 'Content-Length: 2']), xdebug_get_headers());
     }
 
+    /**
+     * @backupGlobals enabled
+     */
+    public function testSendResponseSendsJsonResponseWithGivenHeadersAndMatchingContentLengthButEmptyBodyForHeadRequest()
+    {
+        if (headers_sent() || !function_exists('xdebug_get_headers')) {
+            $this->markTestSkipped('Test requires running phpunit with --stderr and Xdebug enabled');
+        }
+
+        $_SERVER['REQUEST_METHOD'] = 'HEAD';
+        $_SERVER['SERVER_PROTOCOL'] = 'http/1.1';
+        $sapi = new SapiHandler();
+        $response = new Response(200, ['Content-Type' => 'application/json'], '{}');
+
+        $this->expectOutputString('');
+        $sapi->sendResponse($response);
+
+        $previous = ['Content-Type:'];
+        $this->assertEquals(array_merge($previous, ['Content-Type: application/json', 'Content-Length: 2']), xdebug_get_headers());
+    }
+
+    public function testSendResponseSendsEmptyBodyWithGivenHeadersAndAssignsNoContentLengthForNoContentResponse()
+    {
+        if (headers_sent() || !function_exists('xdebug_get_headers')) {
+            $this->markTestSkipped('Test requires running phpunit with --stderr and Xdebug enabled');
+        }
+
+        $_SERVER['SERVER_PROTOCOL'] = 'http/1.1';
+        $sapi = new SapiHandler();
+        $response = new Response(204, ['Content-Type' => 'application/json'], '{}');
+
+        $this->expectOutputString('');
+        $sapi->sendResponse($response);
+
+        $previous = ['Content-Type:', 'Content-Length: 2'];
+        $this->assertEquals(array_merge($previous, ['Content-Type: application/json']), xdebug_get_headers());
+    }
+
+    public function testSendResponseSendsEmptyBodyWithGivenHeadersButWithoutExplicitContentLengthForNoContentResponse()
+    {
+        if (headers_sent() || !function_exists('xdebug_get_headers')) {
+            $this->markTestSkipped('Test requires running phpunit with --stderr and Xdebug enabled');
+        }
+
+        $_SERVER['SERVER_PROTOCOL'] = 'http/1.1';
+        $sapi = new SapiHandler();
+        $response = new Response(204, ['Content-Type' => 'application/json', 'Content-Length' => 2], '{}');
+
+        $this->expectOutputString('');
+        $sapi->sendResponse($response);
+
+        $previous = ['Content-Type:', 'Content-Length: 2'];
+        $this->assertEquals(array_merge($previous, ['Content-Type: application/json']), xdebug_get_headers());
+    }
+
+    public function testSendResponseSendsEmptyBodyWithGivenHeadersAndAssignsContentLengthForNotModifiedResponse()
+    {
+        if (headers_sent() || !function_exists('xdebug_get_headers')) {
+            $this->markTestSkipped('Test requires running phpunit with --stderr and Xdebug enabled');
+        }
+
+        $_SERVER['SERVER_PROTOCOL'] = 'http/1.1';
+        $sapi = new SapiHandler();
+        $response = new Response(304, ['Content-Type' => 'application/json'], 'null');
+
+        $this->expectOutputString('');
+        $sapi->sendResponse($response);
+
+        $previous = ['Content-Type:'];
+        $this->assertEquals(array_merge($previous, ['Content-Type: application/json', 'Content-Length: 4']), xdebug_get_headers());
+    }
+
+    public function testSendResponseSendsEmptyBodyWithGivenHeadersAndExplicitContentLengthForNotModifiedResponse()
+    {
+        if (headers_sent() || !function_exists('xdebug_get_headers')) {
+            $this->markTestSkipped('Test requires running phpunit with --stderr and Xdebug enabled');
+        }
+
+        $_SERVER['SERVER_PROTOCOL'] = 'http/1.1';
+        $sapi = new SapiHandler();
+        $response = new Response(304, ['Content-Type' => 'application/json', 'Content-Length' => '2'], '');
+
+        $this->expectOutputString('');
+        $sapi->sendResponse($response);
+
+        $previous = ['Content-Type:'];
+        $this->assertEquals(array_merge($previous, ['Content-Type: application/json', 'Content-Length: 2']), xdebug_get_headers());
+    }
+
     public function testSendResponseSendsStreamingResponseWithNoHeadersAndBodyFromStreamData()
     {
         if (headers_sent() || !function_exists('xdebug_get_headers')) {
@@ -190,6 +279,67 @@ public function testSendResponseSendsStreamingResponseWithNoHeadersAndBodyFromSt
         $body->end('test');
     }
 
+    /**
+     * @backupGlobals enabled
+     */
+    public function testSendResponseClosesStreamingResponseAndSendsResponseWithNoHeadersAndBodyForHeadRequest()
+    {
+        if (headers_sent() || !function_exists('xdebug_get_headers')) {
+            $this->markTestSkipped('Test requires running phpunit with --stderr and Xdebug enabled');
+        }
+
+        $_SERVER['REQUEST_METHOD'] = 'HEAD';
+        $_SERVER['SERVER_PROTOCOL'] = 'http/1.1';
+        $sapi = new SapiHandler();
+        $body = new ThroughStream();
+        $response = new Response(200, [], $body);
+
+        $this->expectOutputString('');
+        $sapi->sendResponse($response);
+
+        $previous = ['Content-Type:', 'Content-Length: 2', 'Content-Type:'];
+        $this->assertEquals(array_merge($previous, ['Content-Type:']), xdebug_get_headers());
+        $this->assertFalse($body->isReadable());
+    }
+
+    public function testSendResponseClosesStreamingResponseAndSendsResponseWithNoHeadersAndBodyForNotModifiedResponse()
+    {
+        if (headers_sent() || !function_exists('xdebug_get_headers')) {
+            $this->markTestSkipped('Test requires running phpunit with --stderr and Xdebug enabled');
+        }
+
+        $_SERVER['SERVER_PROTOCOL'] = 'http/1.1';
+        $sapi = new SapiHandler();
+        $body = new ThroughStream();
+        $response = new Response(304, [], $body);
+
+        $this->expectOutputString('');
+        $sapi->sendResponse($response);
+
+        $previous = ['Content-Type:', 'Content-Length: 2', 'Content-Type:', 'Content-Type:'];
+        $this->assertEquals(array_merge($previous, ['Content-Type:']), xdebug_get_headers());
+        $this->assertFalse($body->isReadable());
+    }
+
+    public function testSendResponseClosesStreamingResponseAndSendsResponseWithNoHeadersAndBodyForNoContentResponse()
+    {
+        if (headers_sent() || !function_exists('xdebug_get_headers')) {
+            $this->markTestSkipped('Test requires running phpunit with --stderr and Xdebug enabled');
+        }
+
+        $_SERVER['SERVER_PROTOCOL'] = 'http/1.1';
+        $sapi = new SapiHandler();
+        $body = new ThroughStream();
+        $response = new Response(204, [], $body);
+
+        $this->expectOutputString('');
+        $sapi->sendResponse($response);
+
+        $previous = ['Content-Type:', 'Content-Length: 2', 'Content-Type:', 'Content-Type:', 'Content-Type:'];
+        $this->assertEquals(array_merge($previous, ['Content-Type:']), xdebug_get_headers());
+        $this->assertFalse($body->isReadable());
+    }
+
     public function testSendResponseSendsStreamingResponseWithNoHeadersAndBodyFromStreamDataAndNoBufferHeaderForNginxServer()
     {
         if (headers_sent() || !function_exists('xdebug_get_headers')) {
@@ -205,7 +355,7 @@ public function testSendResponseSendsStreamingResponseWithNoHeadersAndBodyFromSt
         $this->expectOutputString('test');
         $sapi->sendResponse($response);
 
-        $previous = ['Content-Type:', 'Content-Length: 2', 'Content-Type:'];
+        $previous = ['Content-Type:', 'Content-Length: 2', 'Content-Type:', 'Content-Type:', 'Content-Type:', 'Content-Type:'];
         $this->assertEquals(array_merge($previous, ['Content-Type:', 'X-Accel-Buffering: no']), xdebug_get_headers());
 
         $body->end('test');
diff --git a/tests/acceptance.sh b/tests/acceptance.sh
index 4607a25..397f376 100755
--- a/tests/acceptance.sh
+++ b/tests/acceptance.sh
@@ -98,11 +98,16 @@ out=$(curl -v $base/method -X DELETE 2>&1);     match "HTTP/.* 200" && match "DE
 out=$(curl -v $base/method -X OPTIONS 2>&1);    match "HTTP/.* 200" && match "OPTIONS"
 out=$(curl -v $base -X OPTIONS --request-target "*" 2>&1);  skipif "Server: nginx" && match "HTTP/.* 200" # skip nginx (400)
 
+out=$(curl -v $base/etag/ 2>&1);                            match "HTTP/.* 200" && match -iP "Content-Length: 0[\r\n]" && match -iP "Etag: \"_\""
+out=$(curl -v $base/etag/ -H 'If-None-Match: "_"' 2>&1);    skipif "Server: ReactPHP" && match "HTTP/.* 304" && notmatch -i "Content-Length" && match -iP "Etag: \"_\"" # skip built-in webserver (always includes Content-Length : 0)
+out=$(curl -v $base/etag/a 2>&1);                           match "HTTP/.* 200" && match -iP "Content-Length: 2[\r\n]" && match -iP "Etag: \"a\""
+out=$(curl -v $base/etag/a -H 'If-None-Match: "a"' 2>&1);   skipif "Server: ReactPHP" && skipif "Server: Apache" && match "HTTP/.* 304" && match -iP "Content-Length: 2[\r\n]" && match -iP "Etag: \"a\"" # skip built-in webserver (always includes Content-Length: 0) and Apache (no Content-Length)
+
 out=$(curl -v $base/headers -H 'Accept: text/html' 2>&1);   match "HTTP/.* 200" && match "\"Accept\": \"text/html\""
 out=$(curl -v $base/headers -d 'name=Alice' 2>&1);          match "HTTP/.* 200" && match "\"Content-Type\": \"application/x-www-form-urlencoded\"" && match "\"Content-Length\": \"10\""
 out=$(curl -v $base/headers -u user:pass 2>&1);             match "HTTP/.* 200" && match "\"Authorization\": \"Basic dXNlcjpwYXNz\""
 out=$(curl -v $base/headers 2>&1);                          match "HTTP/.* 200" && notmatch -i "\"Content-Type\"" && notmatch -i "\"Content-Length\""
-out=$(curl -v $base/headers -H User-Agent: -H Accept: -H Host: -10 2>&1);   skipif "Server: ReactPHP" && match "HTTP/.* 200" && match "{}" # skip built-in webserver (always includes Host)
+out=$(curl -v $base/headers -H User-Agent: -H Accept: -H Host: -10 2>&1);   match "HTTP/.* 200" && match "{}"
 out=$(curl -v $base/headers -H 'Content-Length: 0' 2>&1);   match "HTTP/.* 200" && match "\"Content-Length\": \"0\""
 out=$(curl -v $base/headers -H 'Empty;' 2>&1);              match "HTTP/.* 200" && match "\"Empty\": \"\""
 out=$(curl -v $base/headers -H 'Content-Type;' 2>&1);       skipif "Server: Apache" && match "HTTP/.* 200" && match "\"Content-Type\": \"\"" # skip Apache (discards empty Content-Type)