diff --git a/tests/test-class-amp-comment-walker.php b/tests/test-class-amp-comment-walker.php
new file mode 100644
index 00000000000..1e13edb8c8b
--- /dev/null
+++ b/tests/test-class-amp-comment-walker.php
@@ -0,0 +1,129 @@
+<?php
+/**
+ * Tests for AMP_Comment_Walker class.
+ *
+ * @package AMP
+ */
+
+/**
+ * Tests for AMP_Comment_Walker class.
+ *
+ * @since 0.7
+ */
+class Test_AMP_Comment_Walker extends WP_UnitTestCase {
+
+	/**
+	 * The comment walker.
+	 *
+	 * @var AMP_Comment_Walker
+	 */
+	public $walker;
+
+	/**
+	 * Setup.
+	 *
+	 * @inheritdoc
+	 */
+	public function setUp() {
+		parent::setUp();
+		$this->walker = new AMP_Comment_Walker();
+	}
+
+	/**
+	 * Test AMP_Comment_Walker::start_el.
+	 *
+	 * @covers AMP_Comment_Walker::start_el()
+	 */
+	public function test_start_el() {
+		$GLOBALS['post'] = $this->factory()->post->create(); // WPCS: global override OK.
+		$output          = '<div></div>';
+		$base_args       = array(
+			'format'      => 'baz',
+			'avatar_size' => 100,
+			'max_depth'   => 5,
+		);
+		$args            = array_merge(
+			$base_args,
+			array(
+				'style' => 'baz',
+			)
+		);
+		$comment         = $this->factory()->comment->create_and_get();
+		$this->walker->start_el( $output, $comment, 0, $args );
+		$this->assertContains( '<li data-sort-time=', $output );
+		$this->assertContains( $comment->comment_ID, $output );
+		$this->assertContains( strval( strtotime( $comment->comment_date ) ), $output );
+
+		$output  = '<div></div>';
+		$args    = array_merge(
+			$base_args,
+			array(
+				'style' => 'div',
+			)
+		);
+		$comment = $this->factory()->comment->create_and_get();
+		$this->walker->start_el( $output, $comment, 0, $args );
+		$this->assertContains( '<div data-sort-time=', $output );
+	}
+
+	/**
+	 * Test AMP_Comment_Walker::paged_walk.
+	 *
+	 * @covers AMP_Comment_Walker::paged_walk()
+	 */
+	public function test_paged_walk() {
+		$GLOBALS['post'] = $this->factory()->post->create(); // WPCS: global override OK.
+		$comments        = $this->get_comments();
+		$args            = array(
+			'format'      => 'div',
+			'style'       => 'baz',
+			'avatar_size' => 100,
+			'max_depth'   => 5,
+		);
+		$output          = $this->walker->paged_walk( $comments, 5, 1, 5, $args );
+
+		foreach ( $comments as $comment ) {
+			$this->assertContains( $comment->comment_author, $output );
+			$this->assertContains( $comment->comment_content, $output );
+		}
+	}
+
+	/**
+	 * Test AMP_Comment_Walker::build_thread_latest_date.
+	 *
+	 * @covers AMP_Comment_Walker::build_thread_latest_date()
+	 */
+	public function test_build_thread_latest_date() {
+		$comments      = $this->get_comments();
+		$reflection    = new ReflectionObject( $this->walker );
+		$tested_method = $reflection->getMethod( 'build_thread_latest_date' );
+		$tested_method->setAccessible( true );
+		$latest_time        = $tested_method->invoke( $this->walker, $comments );
+		$comment_thread_age = $reflection->getProperty( 'comment_thread_age' );
+		$comment_thread_age->setAccessible( true );
+		$comment_thread_value = $comment_thread_age->getValue( $this->walker );
+
+		foreach ( $comments as $comment ) {
+			$this->assertEquals( strtotime( $comment->comment_date ), $comment_thread_value[ $comment->comment_ID ] );
+		}
+
+		$last_comment = end( $comments );
+		$this->assertEquals( strtotime( $last_comment->comment_date ), $latest_time );
+	}
+
+	/**
+	 * Gets comments for tests.
+	 *
+	 * @return array $comments An array of WP_Comment instances.
+	 */
+	public function get_comments() {
+		$comments = array();
+		for ( $i = 0; $i < 5; $i++ ) {
+			$comments[] = $this->factory()->comment->create_and_get( array(
+				'comment_date' => gmdate( 'Y-m-d H:i:s', ( time() + $i ) ), // Ensure each comment has a different date.
+			) );
+		}
+		return $comments;
+	}
+
+}
diff --git a/tests/test-class-amp-comments-sanitizer.php b/tests/test-class-amp-comments-sanitizer.php
new file mode 100644
index 00000000000..d5cfda24882
--- /dev/null
+++ b/tests/test-class-amp-comments-sanitizer.php
@@ -0,0 +1,131 @@
+<?php
+/**
+ * Tests for AMP_Comments_Sanitizer class.
+ *
+ * @package AMP
+ */
+
+/**
+ * Tests for AMP_Comments_Sanitizer class.
+ *
+ * @since 0.7
+ */
+class Test_AMP_Comments_Sanitizer extends WP_UnitTestCase {
+
+	/**
+	 * The tested instance.
+	 *
+	 * @var AMP_Comments_Sanitizer
+	 */
+	public $instance;
+
+	/**
+	 * Representation of the DOM.
+	 *
+	 * @var DOMDocument
+	 */
+	public $dom;
+
+	/**
+	 * Setup.
+	 *
+	 * @inheritdoc
+	 */
+	public function setUp() {
+		parent::setUp();
+		$GLOBALS['post'] = $this->factory()->post->create_and_get(); // WPCS: global override ok.
+		$this->dom       = new DOMDocument();
+		$this->instance  = new AMP_Comments_Sanitizer( $this->dom );
+	}
+
+	/**
+	 * Test AMP_Comments_Sanitizer::sanitize.
+	 *
+	 * @covers AMP_Comments_Sanitizer::sanitize()
+	 */
+	public function test_sanitize() {
+		$form = $this->create_form( 'incorrect-action.php' );
+		$this->instance->sanitize();
+		$on = $form->getAttribute( 'on' );
+		$this->assertNotContains( 'submit:AMP.setState(', $on );
+		$this->assertNotContains( 'submit-error:AMP.setState(', $on );
+		foreach ( $this->get_element_names() as $name ) {
+			$this->assertNotContains( $name, $on );
+		}
+
+		// Use an allowed action.
+		$form = $this->create_form( '/wp-comments-post.php' );
+		$this->instance->sanitize();
+		$on = $form->getAttribute( 'on' );
+		$this->assertContains( 'submit:AMP.setState(', $on );
+		$this->assertContains( 'submit-error:AMP.setState(', $on );
+		foreach ( $this->get_element_names() as $name ) {
+			$this->assertContains( $name, $on );
+		}
+	}
+
+	/**
+	 * Test AMP_Comments_Sanitizer::process_comment_form.
+	 *
+	 * @covers AMP_Comments_Sanitizer::process_comment_form()
+	 */
+	public function test_process_comment_form() {
+		$form          = $this->create_form( '/wp-comments-post.php' );
+		$reflection    = new ReflectionObject( $this->instance );
+		$tested_method = $reflection->getMethod( 'process_comment_form' );
+		$tested_method->setAccessible( true );
+		$tested_method->invoke( $this->instance, $form );
+		$on        = $form->getAttribute( 'on' );
+		$amp_state = $this->dom->getElementsByTagName( 'amp-state' )->item( 0 );
+
+		$this->assertContains( 'submit:AMP.setState(', $on );
+		$this->assertContains( 'submit-error:AMP.setState(', $on );
+		$this->assertContains( 'submit-success:AMP.setState(', $on );
+		$this->assertContains( strval( $GLOBALS['post']->ID ), $on );
+		$this->assertEquals( 'script', $amp_state->firstChild->nodeName );
+
+		foreach ( $this->get_element_names() as $name ) {
+			$this->assertContains( $name, $on );
+			$this->assertContains( $name, $amp_state->nodeValue );
+		}
+		foreach ( $form->getElementsByTagName( 'input' ) as $input ) {
+			$on = $input->getAttribute( 'on' );
+			$this->assertContains( 'change:AMP.setState(', $on );
+			$this->assertContains( strval( $GLOBALS['post']->ID ), $on );
+		}
+	}
+
+	/**
+	 * Creates a form for testing.
+	 *
+	 * @param string $action_value Value of the 'action' attribute.
+	 * @return DomElement $form A form element.
+	 */
+	public function create_form( $action_value ) {
+		$form = $this->dom->createElement( 'form' );
+		$this->dom->appendChild( $form );
+		$form->setAttribute( 'action', $action_value );
+
+		foreach ( $this->get_element_names() as $name ) {
+			$element = $this->dom->createElement( 'input' );
+			$element->setAttribute( 'name', $name );
+			$element->setAttribute( 'value', $GLOBALS['post']->ID );
+			$form->appendChild( $element );
+		}
+		return $form;
+	}
+
+	/**
+	 * Gets the element names to add to the <form>.
+	 *
+	 * @return array An array of strings to add to the <form>.
+	 */
+	public function get_element_names() {
+		return array(
+			'comment_post_ID',
+			'foo',
+			'bar',
+		);
+	}
+
+}