diff --git a/.gitignore b/.gitignore
index d1502b0..3d7e132 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,2 +1,3 @@
 vendor/
 composer.lock
+.phpunit.result.cache
diff --git a/phpunit.xml b/phpunit.xml
new file mode 100644
index 0000000..f31a55d
--- /dev/null
+++ b/phpunit.xml
@@ -0,0 +1,7 @@
+<phpunit bootstrap="vendor/autoload.php">
+	<testsuites>
+		<testsuite name="default">
+			<directory>test</directory>
+		</testsuite>
+	</testsuites>
+</phpunit>
diff --git a/test/TokenizerTest.php b/test/TokenizerTest.php
new file mode 100644
index 0000000..af03490
--- /dev/null
+++ b/test/TokenizerTest.php
@@ -0,0 +1,106 @@
+<?php
+/**
+ * MadLisp language
+ * @link http://madlisp.com/
+ * @copyright Copyright (c) 2020 Pekka Laiho
+ */
+
+use PHPUnit\Framework\TestCase;
+
+use MadLisp\MadLispException;
+use MadLisp\Tokenizer;
+
+class TokenizerTest extends TestCase
+{
+    public function exceptionProvider(): array
+    {
+        return [
+            ['"', 'unterminated string'],
+            ['"\\', 'unterminated string'],
+            ['"\\"', 'unterminated string'],
+            ['"\\ ', "invalid escape sequence \\ "],
+            ['"\\a', "invalid escape sequence \\a"],
+            ['(', 'missing closing )'],
+            ['[', 'missing closing ]'],
+            ['{', 'missing closing }'],
+            ['(()', 'missing closing )'],
+            ['[[]', 'missing closing ]'],
+            ['{{}', 'missing closing }'],
+            [')', 'unexpected closing )'],
+            [']', 'unexpected closing ]'],
+            ['}', 'unexpected closing }'],
+            ['())', 'unexpected closing )'],
+            ['[]]', 'unexpected closing ]'],
+            ['{}}', 'unexpected closing }'],
+        ];
+    }
+
+    /**
+     * Test inputs that throw an exception.
+     * @dataProvider exceptionProvider
+     */
+    public function testException(string $input, string $message)
+    {
+        $this->expectException(MadLispException::class);
+        $this->expectExceptionMessage($message);
+
+        $tokenizer = new Tokenizer();
+        $tokenizer->tokenize($input);
+    }
+
+    public function tokenProvider(): array
+    {
+        return [
+            // Ignored characters
+            ["", []],
+            [" ", []],
+            ["\t", []],
+            ["\n", []],
+            ["\r", []],
+            [":", []],
+            [" \t\n\r: ", []],
+            [" aa\t\n\rbb:\r\ncc\t ", ['aa', 'bb', 'cc']],
+
+            // Comments
+            [";comment", []],
+            ["a;c(o[m{m}e]n)t\nb", ['a', 'b']], // parens inside comment
+            ["a;com\"ment\nb", ['a', 'b']], // quote inside comment
+            ["a;comment\rb", ['a', 'b']], // end with \r
+            ["a;;comment\nb", ['a', 'b']], // two ;
+            ["a;com\nb;ment\rc", ['a', 'b', 'c']], // two comments
+
+            // Parens
+            ["aa(bb[cc{dd}ee]gg)ff", ['aa', '(', 'bb', '[', 'cc', '{', 'dd', '}', 'ee', ']', 'gg', ')', 'ff']],
+            // Special characters: '`~
+            ["aa'bb`cc~dd~ee`gg'ff", ['aa', "'", 'bb', '`', 'cc', '~', 'dd', '~', 'ee', '`', 'gg', "'", 'ff']],
+            // Other non-alphabet characters are symbols
+            ["(aa!@#$%^&*-_=+bb<>,./?\\|cc)", ['(', "aa!@#$%^&*-_=+bb<>,./?\\|cc", ')']],
+
+            // Strings
+            ['"abc"', ['"abc"']],
+            ['aa"bb"cc', ['aa', '"bb"', 'cc']],
+            ['aa"bb;cc"dd', ['aa', '"bb;cc"', 'dd']], // comment inside string
+            ['aa"bb""cc"dd', ['aa', '"bb"', '"cc"', 'dd']], // two strings
+            ["aa\"bb\\\"cc\"dd", ['aa', "\"bb\"cc\"", 'dd']], // quote inside string
+            ["aa\"bb\n\rcc\"dd", ['aa', "\"bb\n\rcc\"", 'dd']], // linebreaks inside string
+            ["aa\"bb\\n\\r\\tcc\"dd", ['aa', "\"bb\n\r\tcc\"", 'dd']], // escaped linebreaks
+            ["aa\"bb\\\\n\\\\rcc\"dd", ['aa', "\"bb\\n\\rcc\"", 'dd']], // escaped backslashes
+            ["aa\"bb\\\\\"cc", ['aa', "\"bb\\\"", 'cc']],
+            ["aa\"bb\\\\\\\"cc\"dd", ['aa', "\"bb\\\"cc\"", 'dd']],
+
+            // Test everything together
+            ["(abc<+=-_!?>\"str\n\\r;\\\"\";com\"ment\r{\"a\":\"b\"})", ['(', 'abc<+=-_!?>', "\"str\n\r;\"\"", '{', '"a"', '"b"', '}', ')']],
+        ];
+    }
+
+    /**
+     * Test valid inputs.
+     * @dataProvider tokenProvider
+     */
+    public function testTokenize(string $input, array $expected)
+    {
+        $tokenizer = new Tokenizer();
+        $result = $tokenizer->tokenize($input);
+        $this->assertSame($expected, $result);
+    }
+}