diff --git a/tests/phpunit/tests/Debug/Debug_LogTest.php b/tests/phpunit/tests/Debug/Debug_LogTest.php
new file mode 100644
index 0000000..c95036f
--- /dev/null
+++ b/tests/phpunit/tests/Debug/Debug_LogTest.php
@@ -0,0 +1,210 @@
+<?php
+/**
+ * Class Debug_LogTest
+ *
+ * @package AspireUpdate
+ */
+
+/**
+ * Tests for Debug::log()
+ *
+ * @covers \AspireUpdate\Debug::log
+ */
+class Debug_LogTest extends Debug_UnitTestCase {
+	/**
+	 * Test that nothing is written to the log file when the filesystem isn't available.
+	 *
+	 * @covers \AspireUpdate\Debug::init_filesystem
+	 * @covers \AspireUpdate\Debug::verify_filesystem
+	 */
+	public function test_should_not_write_to_log_file_when_filesystem_is_not_available() {
+		add_filter( 'filesystem_method', '__return_false' );
+
+		AspireUpdate\Debug::log( 'Test log message.' );
+
+		$this->assertFileDoesNotExist(
+			self::$log_file,
+			'The log file was created.'
+		);
+	}
+
+	/**
+	 * Test that the log file is created when it doesn't already exist.
+	 */
+	public function test_should_create_log_file_if_it_does_not_already_exist() {
+		$this->assertFileDoesNotExist(
+			self::$log_file,
+			'The log file already exists before testing.'
+		);
+
+		$message = 'Test log message.';
+
+		AspireUpdate\Debug::log( $message );
+
+		$this->assertFileExists(
+			self::$log_file,
+			'The log file was not created.'
+		);
+	}
+
+	/**
+	 * Test that the message is added to the log file.
+	 *
+	 * @covers \AspireUpdate\Debug::format_message
+	 */
+	public function test_should_add_message_to_log_file() {
+		$this->assertFileDoesNotExist(
+			self::$log_file,
+			'The log file already exists before testing.'
+		);
+
+		$message = 'Test log message.';
+
+		AspireUpdate\Debug::log( $message );
+
+		$this->assertFileExists(
+			self::$log_file,
+			'The log file was not created.'
+		);
+
+		$this->assertStringContainsString(
+			$message,
+			file_get_contents( self::$log_file ),
+			'The message was not added.'
+		);
+	}
+
+	/**
+	 * Test that the message is prepended to an existing log file.
+	 *
+	 * @covers \AspireUpdate\Debug::format_message
+	 */
+	public function test_should_add_message_to_an_existing_log_file() {
+		$existing_content = 'An existing log file.';
+		file_put_contents( self::$log_file, $existing_content );
+
+		$message = 'Test log message.';
+
+		AspireUpdate\Debug::log( $message );
+
+		$this->assertFileExists(
+			self::$log_file,
+			'The log file was not created.'
+		);
+
+		$this->assertStringContainsString(
+			"$message\n$existing_content",
+			file_get_contents( self::$log_file ),
+			'The message was not prepended to the log file.'
+		);
+	}
+
+	/**
+	 * Test that the message is prefixed with the timestamp.
+	 *
+	 * @covers \AspireUpdate\Debug::format_message
+	 */
+	public function test_should_prefix_message_with_timestamp() {
+		AspireUpdate\Debug::log( 'Test log message.' );
+
+		$this->assertFileExists(
+			self::$log_file,
+			'The log file was not created.'
+		);
+
+		$this->assertMatchesRegularExpression(
+			'/^\[[0-9]{4}\-[0-9]{2}\-[0-9]{2} [0-9]{2}:[0-9]{2}:[0-9]{2}\]/',
+			file_get_contents( self::$log_file ),
+			'The message was not prefixed with the timestamp.'
+		);
+	}
+
+	/**
+	 * Test that the message is prefixed with its type.
+	 *
+	 * @dataProvider data_message_types
+	 *
+	 * @covers \AspireUpdate\Debug::format_message
+	 *
+	 * @param string $type The type of message.
+	 */
+	public function test_should_prefix_message_with_type( $type ) {
+		$message = 'Test log message.';
+
+		AspireUpdate\Debug::log( $message, $type );
+
+		$this->assertFileExists(
+			self::$log_file,
+			'The log file was not created.'
+		);
+
+		$this->assertStringContainsString(
+			'[' . strtoupper( $type ) . ']: ' . $message,
+			file_get_contents( self::$log_file ),
+			'The message was not prefixed with its type.'
+		);
+	}
+
+	/**
+	 * Data provider.
+	 *
+	 * @return array[]
+	 */
+	public function data_message_types() {
+		return $this->text_array_to_dataprovider(
+			[
+				'string',
+				'request',
+				'response',
+				'custom',
+			]
+		);
+	}
+
+	/**
+	 * Test that array and object messages are expanded.
+	 *
+	 * @dataProvider data_arrays_and_objects
+	 *
+	 * @covers \AspireUpdate\Debug::format_message
+	 *
+	 * @param array|object $message The message.
+	 */
+	public function test_should_expand_array_or_object_messages( $message ) {
+		AspireUpdate\Debug::log( $message );
+
+		$this->assertFileExists(
+			self::$log_file,
+			'The log file was not created.'
+		);
+
+		$this->assertStringContainsString(
+			// phpcs:ignore WordPress.PHP.DevelopmentFunctions.error_log_print_r
+			print_r( $message, true ),
+			file_get_contents( self::$log_file ),
+			'The array message was not expanded.'
+		);
+	}
+
+	/**
+	 * Data provider.
+	 *
+	 * @return array[]
+	 */
+	public function data_arrays_and_objects() {
+		return [
+			'an array'                     => [
+				'message' => [],
+			],
+			'a non-empty array'            => [
+				'message' => [ 'First line', 'Second line', 'Third line' ],
+			],
+			'an object with no properties' => [
+				'message' => (object) [],
+			],
+			'an object with properties'    => [
+				'message' => (object) [ 'First line', 'Second line', 'Third line' ],
+			],
+		];
+	}
+}