diff --git a/.gitignore b/.gitignore
index 8f318fb..9d55128 100644
--- a/.gitignore
+++ b/.gitignore
@@ -3,3 +3,4 @@
 /test/phpunit/_coverage
 /*.phar
 .phpunit.*.cache
+/test/phpunit/.phpunit.cache/
diff --git a/src/Header/HeaderLine.php b/src/Header/HeaderLine.php
index f9afc69..078cfb0 100644
--- a/src/Header/HeaderLine.php
+++ b/src/Header/HeaderLine.php
@@ -17,7 +17,7 @@ public function __construct(string $name, string...$values) {
 	}
 
 	public function __toString():string {
-		if(in_array($this->name, Headers::COMMA_HEADERS)) {
+		if(in_array($this->name, Headers::NON_COMBINABLE_HEADERS)) {
 			return $this->getValuesNewlineSeparated();
 		}
 		else {
diff --git a/src/Header/Headers.php b/src/Header/Headers.php
index 6d8ee3c..f7f2835 100644
--- a/src/Header/Headers.php
+++ b/src/Header/Headers.php
@@ -13,11 +13,8 @@
 class Headers implements Iterator, Countable, TypeSafeGetter {
 	use NullableTypeSafeGetter;
 
-	const COMMA_HEADERS = [
-// These cookies use commas within the value, so can't be comma separated.
-		"cookie-set",
-		"www-authenticate",
-		"proxy-authenticate"
+	const NON_COMBINABLE_HEADERS = [
+		"set-cookie",
 	];
 
 	/** @var HeaderLine[] */
@@ -43,17 +40,29 @@ public function asArray(bool $nested = false):array {
 
 		foreach($this->headerLines as $header) {
 			$name = $header->getName();
+			$nameLower = strtolower($name);
 
 			if($nested) {
-				$array[$name] = $header->getValues();
+				$array[$name] ??= [];
+				$array[$name] = array_merge($array[$name], $header->getValues());
 				continue;
 			}
 
-			if(in_array(strtolower($name), self::COMMA_HEADERS)) {
-				$array[$name] = $header->getValuesNewlineSeparated();
+			if(!array_key_exists($name, $array)) {
+				$array[$name] = "";
+			}
+
+			if(in_array($nameLower, self::NON_COMBINABLE_HEADERS)) {
+				if($array[$name] !== "") {
+					$array[$name] .= "\n";
+				}
+				$array[$name] .= $header->getValuesNewlineSeparated();
 			}
 			else {
-				$array[$name] = $header->getValuesCommaSeparated();
+				if($array[$name] !== "") {
+					$array[$name] .= ",";
+				}
+				$array[$name] .= $header->getValuesCommaSeparated();
 			}
 		}
 
@@ -67,7 +76,14 @@ public function fromArray(array $headerArray):void {
 				$value = [$value];
 			}
 
-			$this->headerLines []= new HeaderLine($key, ...$value);
+			if(in_array(strtolower($key), self::NON_COMBINABLE_HEADERS)) {
+				foreach($value as $singleValue) {
+					array_push($this->headerLines, new HeaderLine($key, $singleValue));
+				}
+			}
+			else {
+				array_push($this->headerLines, new HeaderLine($key, ...$value));
+			}
 		}
 	}
 
@@ -82,10 +98,11 @@ public function contains(string $name):bool {
 	}
 
 	public function add(string $name, string...$values):void {
-		$isCommaHeader = false;
-		if(strstr($values[0], ",")
-		&& in_array(strtolower($name), self::COMMA_HEADERS)) {
-			$isCommaHeader = true;
+		if(in_array(strtolower($name), self::NON_COMBINABLE_HEADERS)) {
+			foreach($values as $value) {
+				array_push($this->headerLines, new HeaderLine($name, $value));
+			}
+			return;
 		}
 
 		$headerLineToAdd = null;
@@ -97,7 +114,7 @@ public function add(string $name, string...$values):void {
 			$headerLineToAdd = $headerLine;
 		}
 
-		if(is_null($headerLineToAdd) || $isCommaHeader) {
+		if(is_null($headerLineToAdd)) {
 			array_push(
 				$this->headerLines,
 				new HeaderLine($name, ...$values)
@@ -123,24 +140,34 @@ public function remove(string $name):void {
 	}
 
 	public function get(string $name):?HeaderLine {
+		$matchingValues = [];
+		$headerName = null;
+
 		foreach($this->headerLines as $line) {
 			if($line->isNamed($name)) {
-				return $line;
+				$headerName ??= $line->getName();
+				$matchingValues = array_merge($matchingValues, $line->getValues());
 			}
 		}
 
-		return null;
+		if(!$headerName) {
+			return null;
+		}
+
+		return new HeaderLine($headerName, ...$matchingValues);
 	}
 
-	/** @return null|array<int, string> */
-	public function getAll(string $name):?array {
+	/** @return array<int, string> */
+	public function getAll(string $name):array {
+		$allValues = [];
+
 		foreach($this->headerLines as $line) {
 			if($line->isNamed($name)) {
-				return $line->getValues();
+				$allValues = array_merge($allValues, $line->getValues());
 			}
 		}
 
-		return null;
+		return $allValues;
 	}
 
 	public function getFirst():string {
diff --git a/src/Header/Parser.php b/src/Header/Parser.php
index 7a259e5..8a2b971 100644
--- a/src/Header/Parser.php
+++ b/src/Header/Parser.php
@@ -31,8 +31,20 @@ public function getKeyValues():array {
 			$kvp = explode(":", $h, 2);
 			$key = $kvp[0];
 			$value = $kvp[1] ?? "";
+			$value = trim($value);
 
-			$keyValues[$key] = trim($value);
+			if(array_key_exists($key, $keyValues)) {
+				if(strtolower($key) === "set-cookie") {
+					$keyValues[$key] .= "\n" . $value;
+				}
+				else {
+					$keyValues[$key] .= ", " . $value;
+				}
+
+				continue;
+			}
+
+			$keyValues[$key] = $value;
 		}
 
 		return $keyValues;
@@ -50,6 +62,6 @@ protected function pregMatchProtocol(string $matchName):string {
 			return "";
 		}
 
-		return (string)($matches[$matchName] ?? "");
+		return $matches[$matchName] ?? "";
 	}
 }
diff --git a/test/phpunit/Header/HeadersTest.php b/test/phpunit/Header/HeadersTest.php
index 17e7b28..356f822 100644
--- a/test/phpunit/Header/HeadersTest.php
+++ b/test/phpunit/Header/HeadersTest.php
@@ -62,12 +62,12 @@ public function testAddMultiple() {
 	public function testAddMultipleCommaHeader() {
 		$headers = new Headers(self::HEADER_ARRAY);
 		$headers->add(
-			"Cookie-set",
+			"Set-Cookie",
 			"language=en; expires=Thu, 1-Jan-1970 00:00:00 UTC; path=/; domain=example.com",
 			"id=123; expires=Thu, 1-Jan-1970 00:00:00 UTC; path=/; domain=example.com httponly"
 		);
 		$headerArray = $headers->asArray();
-		$cookie = explode("\n", $headerArray["Cookie-set"]);
+		$cookie = explode("\n", $headerArray["Set-Cookie"]);
 		self::assertContains("language=en; expires=Thu, 1-Jan-1970 00:00:00 UTC; path=/; domain=example.com", $cookie);
 		self::assertContains("id=123; expires=Thu, 1-Jan-1970 00:00:00 UTC; path=/; domain=example.com httponly", $cookie);
 	}
@@ -115,7 +115,7 @@ public function testGetMultiple() {
 	public function testGetMultipleCommas() {
 		$headers = new Headers(self::HEADER_ARRAY);
 		$headers->add(
-			"Cookie-set",
+			"Set-Cookie",
 			"language=en; expires=Thu, 1-Jan-1970 00:00:00 UTC; path=/; domain=example.com",
 			"id=123; expires=Thu, 1-Jan-1970 00:00:00 UTC; path=/; domain=example.com httponly"
 		);
@@ -123,7 +123,58 @@ public function testGetMultipleCommas() {
 			"language=en; expires=Thu, 1-Jan-1970 00:00:00 UTC; path=/; domain=example.com"
 			. "\n"
 			. "id=123; expires=Thu, 1-Jan-1970 00:00:00 UTC; path=/; domain=example.com httponly",
-			$headers->get("Cookie-set")
+			$headers->get("Set-Cookie")
+		);
+	}
+
+	public function testAddMultipleSetCookieWithoutCommaPreservesSeparateLines():void {
+		$headers = new Headers(self::HEADER_ARRAY);
+		$headers->add("Set-Cookie", "language=en; Path=/");
+		$headers->add("Set-Cookie", "id=123; HttpOnly");
+
+		self::assertCount(5, $headers);
+		self::assertSame(
+			"language=en; Path=/\nid=123; HttpOnly",
+			$headers->asArray()["Set-Cookie"]
+		);
+	}
+
+	public function testGetAllAggregatesValuesAcrossSeparateSetCookieHeaderLines():void {
+		$firstValue = "language=en; Path=/";
+		$secondValue = "id=123; HttpOnly";
+		$headers = new Headers(self::HEADER_ARRAY);
+		$headers->add("Set-Cookie", $firstValue);
+		$headers->add("Set-Cookie", $secondValue);
+
+		self::assertSame(
+			[$firstValue, $secondValue],
+			$headers->getAll("Set-Cookie")
+		);
+	}
+
+	public function testAsArrayPreservesAllDuplicateSetCookieHeaderLines():void {
+		$firstValue = "language=en; Path=/";
+		$secondValue = "id=123; HttpOnly";
+		$headers = new Headers(self::HEADER_ARRAY);
+		$headers->add("Set-Cookie", $firstValue);
+		$headers->add("Set-Cookie", $secondValue);
+
+		self::assertSame(
+			$firstValue . "\n" . $secondValue,
+			$headers->asArray()["Set-Cookie"]
+		);
+	}
+
+	public function testAsArrayNestedPreservesAllDuplicateSetCookieHeaderLineValues():void {
+		$firstValue = "language=en; Path=/";
+		$secondValue = "id=123; HttpOnly";
+		$headers = new Headers(self::HEADER_ARRAY);
+		$headers->add("Set-Cookie", $firstValue);
+		$headers->add("Set-Cookie", $secondValue);
+
+		self::assertSame(
+			[$firstValue, $secondValue],
+			$headers->asArray(true)["Set-Cookie"]
 		);
 	}
 
diff --git a/test/phpunit/Header/ParserTest.php b/test/phpunit/Header/ParserTest.php
index 705e337..faa9bc6 100644
--- a/test/phpunit/Header/ParserTest.php
+++ b/test/phpunit/Header/ParserTest.php
@@ -73,4 +73,18 @@ public function testGetKeyValuesResponse() {
 		self::assertArrayHasKey("X-Test-For", $keyValues);
 		self::assertEquals("PHP.Gt", $keyValues["X-Test-For"]);
 	}
+
+	public function testGetKeyValuesCombinesRepeatedListValuedHeaders():void {
+		$headers = <<<HEADERS
+HTTP/1.1 200 OK
+Accept: text/html
+Accept: application/json
+HEADERS;
+		$parser = new Parser($headers);
+
+		self::assertSame(
+			"text/html, application/json",
+			$parser->getKeyValues()["Accept"]
+		);
+	}
 }
diff --git a/test/phpunit/RequestTest.php b/test/phpunit/RequestTest.php
index b833791..8f0bddb 100644
--- a/test/phpunit/RequestTest.php
+++ b/test/phpunit/RequestTest.php
@@ -115,6 +115,26 @@ public function testWithFormDataBodyAutomaticallySetsContentTypeHeader():void {
 		);
 	}
 
+	public function testGetHeaderReturnsAllValuesAcrossSeparateHeaderLines():void {
+		$headers = new RequestHeaders();
+		$headers->add("Set-Cookie", "language=en; Path=/");
+		$headers->add("Set-Cookie", "id=123; HttpOnly");
+
+		$request = new Request(
+			"GET",
+			self::getUriMock("/"),
+			$headers
+		);
+
+		self::assertSame(
+			[
+				"language=en; Path=/",
+				"id=123; HttpOnly",
+			],
+			$request->getHeader("Set-Cookie")
+		);
+	}
+
 	/** @return MockObject|Uri */
 	protected function getUriMock(string $uriPath = ""):MockObject {
 		$partPath = parse_url($uriPath, PHP_URL_PATH);