diff --git a/.github/workflows/phpunit-tests.yml b/.github/workflows/phpunit-tests.yml
index 9aabbb7..eef2b75 100644
--- a/.github/workflows/phpunit-tests.yml
+++ b/.github/workflows/phpunit-tests.yml
@@ -35,8 +35,16 @@ jobs:
           php-version: ${{ matrix.php-version }}
           tools: phpunit-polyfills:1.1
 
+      - name: Install SVN
+        run: sudo apt install -y subversion
+
       - name: Setup tests
         run: bash bin/install-wp-tests.sh wordpress_tests root root 127.0.0.1 latest true
 
+      - name: Prime the WordPress install for tests
+        id: prime-tests
+        run: XDEBUG_MODE=off phpunit${{ matrix.multisite && ' -c tests/phpunit/multisite.xml' || '' }} --filter gha_install_wp > /dev/null 2>&1
+
       - name: Run tests
-        run: XDEBUG_MODE=off phpunit${{ matrix.multisite && ' -c tests/phpunit/multisite.xml' || '' }}
+        if: steps.prime-tests.outcome == 'success'
+        run: XDEBUG_MODE=off WP_TESTS_SKIP_INSTALL=1 phpunit${{ matrix.multisite && ' -c tests/phpunit/multisite.xml' || '' }}
diff --git a/includes/class-admin-settings.php b/includes/class-admin-settings.php
index 414b604..b075cc3 100644
--- a/includes/class-admin-settings.php
+++ b/includes/class-admin-settings.php
@@ -506,6 +506,8 @@ public function register_settings() {
 	/**
 	 * The Fields API which any CMS should have in its core but something we dont, hence this ugly hack.
 	 *
+	 * @codeCoverageIgnore Test with E2E tests instead.
+	 *
 	 * @param array $args The Field Parameters.
 	 *
 	 * @return void Echos the Field HTML.
diff --git a/includes/class-debug.php b/includes/class-debug.php
index eba51fd..81ab276 100644
--- a/includes/class-debug.php
+++ b/includes/class-debug.php
@@ -63,7 +63,7 @@ private static function verify_filesystem( $wp_filesystem ) {
 				/**
 				 * Log error in file write fails only if debug is set to true.  This is a valid use case.
 				 */
-				error_log( 'AspireUpdate - Could not open or write to the file system. Check file system permissions to debug log directory.' );
+				error_log( 'AspireUpdate - Could not open or write to the file system. Check file system permissions to debug log directory.' ); // @codeCoverageIgnore
 				// phpcs:enable
 			}
 			return false;
diff --git a/phpcs.xml.dist b/phpcs.xml.dist
index 081e4b7..66ac36d 100644
--- a/phpcs.xml.dist
+++ b/phpcs.xml.dist
@@ -61,9 +61,25 @@
 
 	<!-- Exclude test classes from naming conventions. -->
 	<rule ref="WordPress.Files.FileName.InvalidClassFileName">
-		<exclude-pattern>/tests/phpunit/tests/*</exclude-pattern>
+		<exclude-pattern>/tests/phpunit/*</exclude-pattern>
 	</rule>
 	<rule ref="WordPress.Files.FileName.NotHyphenatedLowercase">
+		<exclude-pattern>/tests/phpunit/*</exclude-pattern>
+	</rule>
+
+	<!-- Exclude test files from overriding globals sniff. -->
+	<rule ref="WordPress.WP.GlobalVariablesOverride.Prohibited">
 		<exclude-pattern>/tests/phpunit/tests/*</exclude-pattern>
 	</rule>
+
+	<!-- Exclude test files from some filesystem sniffs. -->
+	<rule ref="WordPress.WP.AlternativeFunctions.file_get_contents_file_get_contents">
+		<exclude-pattern>/tests/phpunit/*</exclude-pattern>
+	</rule>
+	<rule ref="WordPress.WP.AlternativeFunctions.file_system_operations_file_put_contents">
+		<exclude-pattern>/tests/phpunit/*</exclude-pattern>
+	</rule>
+	<rule ref="WordPress.WP.AlternativeFunctions.unlink_unlink">
+		<exclude-pattern>/tests/phpunit/*</exclude-pattern>
+	</rule>
 </ruleset>
diff --git a/tests/phpunit/bootstrap.php b/tests/phpunit/bootstrap.php
index 3899953..186c80a 100644
--- a/tests/phpunit/bootstrap.php
+++ b/tests/phpunit/bootstrap.php
@@ -40,5 +40,9 @@ function _manually_load_plugin() {
 // Start up the WP testing environment.
 require "{$_tests_dir}/includes/bootstrap.php";
 
+// Load unit test mocks and fakes.
+require __DIR__ . '/includes/class-ap-fake-filesystem.php';
+
 // Load unit test abstract classes.
 require __DIR__ . '/includes/AdminSettings_UnitTestCase.php';
+require __DIR__ . '/includes/Debug_UnitTestCase.php';
diff --git a/tests/phpunit/includes/Debug_UnitTestCase.php b/tests/phpunit/includes/Debug_UnitTestCase.php
new file mode 100644
index 0000000..0058fd0
--- /dev/null
+++ b/tests/phpunit/includes/Debug_UnitTestCase.php
@@ -0,0 +1,116 @@
+<?php
+/**
+ * Abstract base test class for \AspireUpdate\Debug.
+ *
+ * All \AspireUpdate\Debug unit tests should inherit from this class.
+ */
+abstract class Debug_UnitTestCase extends WP_UnitTestCase {
+	/**
+	 * The path to the log file.
+	 *
+	 * @var string
+	 */
+	protected static $log_file;
+
+	/**
+	 * Previously created filesystems.
+	 *
+	 * @var array
+	 */
+	protected static $filesystems = [];
+
+	/**
+	 * The original value of $wp_filesystem before any tests run.
+	 *
+	 * @var WP_Filesystem_Base|null|false False if not already set.
+	 */
+	protected static $default_filesystem;
+
+	/**
+	 * Gets the log file's path, and deletes if it exists before any tests run.
+	 * Backs up the default filesystem.
+	 *
+	 * @return void
+	 */
+	public static function set_up_before_class() {
+		parent::set_up_before_class();
+
+		$get_file_path = new ReflectionMethod(
+			'AspireUpdate\Debug',
+			'get_file_path'
+		);
+
+		$get_file_path->setAccessible( true );
+		self::$log_file = $get_file_path->invoke( null );
+		$get_file_path->setAccessible( false );
+
+		if ( file_exists( self::$log_file ) ) {
+			unlink( self::$log_file );
+		}
+
+		if ( isset( $GLOBALS['wp_filesystem'] ) ) {
+			self::$default_filesystem = $GLOBALS['wp_filesystem'];
+		} else {
+			self::$default_filesystem = false;
+		}
+	}
+
+	/**
+	 * Filters the filesystem method before each test runs.
+	 *
+	 * Filters are removed in the tear_down() parent method.
+	 *
+	 * @return void
+	 */
+	public function set_up() {
+		parent::set_up();
+
+		add_filter(
+			'filesystem_method',
+			static function () {
+				return 'direct';
+			}
+		);
+	}
+
+	/**
+	 * Delete the log file and restores the filesystem after each test runs.
+	 *
+	 * @return void
+	 */
+	public function tear_down() {
+		if ( file_exists( self::$log_file ) ) {
+			unlink( self::$log_file );
+		}
+
+		if ( false === self::$default_filesystem ) {
+			unset( $GLOBALS['wp_filesystem'] );
+		} else {
+			$GLOBALS['wp_filesystem'] = self::$default_filesystem;
+		}
+
+		parent::tear_down();
+	}
+
+	/**
+	 * Creates a fake filesystem.
+	 *
+	 * @param bool|null $exists      Whether paths should exist.
+	 *                               Default null uses default implemenation.
+	 * @param bool|null $is_readable Whether paths should be readable.
+	 *                               Default null uses default implemenation.
+	 * @param bool|null $is_writable Whether paths should be writable.
+	 *                               Default null uses default implemenation.
+	 */
+	public function get_fake_filesystem( $exists = null, $is_readable = null, $is_writable = null ) {
+		$hash = ( null === $exists ? '-1' : (int) $exists ) . ',' .
+				( null === $is_readable ? '-1' : (int) $is_readable ) . ',' .
+				( null === $is_writable ? '-1' : (int) $is_writable );
+
+		if ( ! isset( self::$filesystems[ $hash ] ) ) {
+			self::$filesystems[ $hash ] = new AP_FakeFilesystem( $exists, $is_readable, $is_writable );
+		}
+
+		return self::$filesystems[ $hash ];
+	}
+}
diff --git a/tests/phpunit/includes/class-ap-fake-filesystem.php b/tests/phpunit/includes/class-ap-fake-filesystem.php
new file mode 100644
index 0000000..d2de759
--- /dev/null
+++ b/tests/phpunit/includes/class-ap-fake-filesystem.php
@@ -0,0 +1,82 @@
+<?php
+
+require_once ABSPATH . 'wp-admin/includes/class-wp-filesystem-base.php';
+require_once ABSPATH . 'wp-admin/includes/class-wp-filesystem-direct.php';
+
+class AP_FakeFilesystem extends WP_Filesystem_Direct {
+	/**
+	 * Whether paths should exist.
+	 *
+	 * @var bool|null
+	 */
+	private $exists;
+
+	/**
+	 * Whether paths should be readable.
+	 *
+	 * @var bool|null
+	 */
+	private $is_readable;
+
+	/**
+	 * Whether paths should be writable.
+	 *
+	 * @var bool|null
+	 */
+	private $is_writable;
+
+	/**
+	 * Contructor.
+	 *
+	 * @param bool|null $exists      Whether paths should exist.
+	 *                               Default null to use parent implementation.
+	 * @param bool|null $is_readable Whether paths should be readable.
+	 *                               Default null to use parent implementation.
+	 * @param bool|null $is_writable Whether paths should be writable.
+	 *                               Default null to use parent implementation.
+	 */
+	public function __construct( $exists = null, $is_readable = null, $is_writable = null ) {
+		$this->exists      = $exists;
+		$this->is_readable = $is_readable;
+		$this->is_writable = $is_writable;
+	}
+
+	/**
+	 * Checks whether a path exists.
+	 *
+	 * @param string $path The path to check.
+	 * @return bool Whether the path exists.
+	 */
+	public function exists( $path ) {
+		if ( null === $this->exists ) {
+			return parent::exists( $path );
+		}
+		return $this->exists;
+	}
+
+	/**
+	 * Checks whether a path is readable.
+	 *
+	 * @param string $path The path to check.
+	 * @return bool Whether the path is readable.
+	 */
+	public function is_readable( $path ) {
+		if ( null === $this->is_readable ) {
+			return parent::is_readable( $path );
+		}
+		return $this->is_readable;
+	}
+
+	/**
+	 * Checks whether a path is writable.
+	 *
+	 * @param string $path The path to check.
+	 * @return bool Whether the path is writable.
+	 */
+	public function is_writable( $path ) {
+		if ( null === $this->is_writable ) {
+			return parent::is_writable( $path );
+		}
+		return $this->is_writable;
+	}
+}
diff --git a/tests/phpunit/tests/Debug/Debug_ClearTest.php b/tests/phpunit/tests/Debug/Debug_ClearTest.php
new file mode 100644
index 0000000..d6882c9
--- /dev/null
+++ b/tests/phpunit/tests/Debug/Debug_ClearTest.php
@@ -0,0 +1,106 @@
+<?php
+/**
+ * Class Debug_ClearTest
+ *
+ * @package AspireUpdate
+ */
+
+/**
+ * Tests for Debug::clear()
+ *
+ * @covers \AspireUpdate\Debug::clear
+ */
+class Debug_ClearTest extends Debug_UnitTestCase {
+	/**
+	 * Test that a WP_Error object is returned when the filesystem isn't available.
+	 *
+	 * @covers \AspireUpdate\Debug::init_filesystem
+	 * @covers \AspireUpdate\Debug::verify_filesystem
+	 */
+	public function test_should_return_wp_error_when_filesystem_is_not_available() {
+		add_filter( 'filesystem_method', '__return_false' );
+
+		$this->assertWPError(
+			AspireUpdate\Debug::clear(),
+			'A WP_Error object was not returned.'
+		);
+
+		$this->assertFileDoesNotExist(
+			self::$log_file,
+			'The log file was created.'
+		);
+	}
+
+	/**
+	 * Test that a WP_Error object is returned when the log file doesn't exist.
+	 *
+	 * @covers \AspireUpdate\Debug::init_filesystem
+	 * @covers \AspireUpdate\Debug::verify_filesystem
+	 * @covers \AspireUpdate\Debug::get_file_path
+	 */
+	public function test_should_return_wp_error_when_log_file_does_not_exist() {
+		$this->assertWPError(
+			AspireUpdate\Debug::clear(),
+			'A WP_Error object was not returned.'
+		);
+
+		$this->assertFileDoesNotExist(
+			self::$log_file,
+			'The log file was created.'
+		);
+	}
+
+	/**
+	 * Test that a WP_Error object is returned when the log file isn't writable.
+	 *
+	 * @covers \AspireUpdate\Debug::init_filesystem
+	 * @covers \AspireUpdate\Debug::verify_filesystem
+	 * @covers \AspireUpdate\Debug::get_file_path
+	 */
+	public function test_should_return_wp_error_when_log_file_is_not_writable() {
+		global $wp_filesystem;
+
+		// Backup and replace the filesystem object.
+		$wp_filesystem = $this->get_fake_filesystem( true, true, false );
+
+		$actual = AspireUpdate\Debug::clear();
+
+		$this->assertWPError(
+			$actual,
+			'A WP_Error was not returned.'
+		);
+
+		$this->assertFileDoesNotExist(
+			self::$log_file,
+			'The log file was created.'
+		);
+	}
+
+	/**
+	 * Test that the log file is cleared.
+	 */
+	public function test_should_clear_log_file() {
+		file_put_contents(
+			self::$log_file,
+			"First line\r\nSecond line\r\nThird line"
+		);
+
+		$this->assertFileExists(
+			self::$log_file,
+			'The log file was not created before testing.'
+		);
+
+		AspireUpdate\Debug::clear();
+
+		$this->assertFileExists(
+			self::$log_file,
+			'The log file was deleted.'
+		);
+
+		$this->assertSame(
+			'',
+			file_get_contents( self::$log_file ),
+			'The log file was not cleared.'
+		);
+	}
+}
diff --git a/tests/phpunit/tests/Debug/Debug_LogRequestTest.php b/tests/phpunit/tests/Debug/Debug_LogRequestTest.php
new file mode 100644
index 0000000..e16ba57
--- /dev/null
+++ b/tests/phpunit/tests/Debug/Debug_LogRequestTest.php
@@ -0,0 +1,104 @@
+<?php
+/**
+ * Class Debug_LogRequestTest
+ *
+ * @package AspireUpdate
+ */
+
+/**
+ * Tests for Debug::log_request()
+ *
+ * These tests cause constants to be defined.
+ * They must run in separate processes and must not preserve global state.
+ *
+ * @runTestsInSeparateProcesses
+ * @preserveGlobalState disabled
+ *
+ * @covers \AspireUpdate\Debug::log_request
+ */
+class Debug_LogRequestTest extends Debug_UnitTestCase {
+	/**
+	 * Test that nothing is written to the log file when debugging is disabled.
+	 */
+	public function test_should_not_write_to_log_file_when_debugging_is_disabled() {
+		define( 'AP_DEBUG', false );
+		define( 'AP_DEBUG_TYPES', [ 'request', 'response', 'string' ] );
+
+		AspireUpdate\Debug::log_request( 'Test log message.' );
+
+		$this->assertFileDoesNotExist( self::$log_file );
+	}
+
+	/**
+	 * Test that nothing is written to the log file when debug types are not an array.
+	 */
+	public function test_should_not_write_to_log_file_when_debug_types_are_not_an_array() {
+		define( 'AP_DEBUG', true );
+		define( 'AP_DEBUG_TYPES', 'request' );
+
+		AspireUpdate\Debug::log_request( 'Test log message.' );
+
+		$this->assertFileDoesNotExist( self::$log_file );
+	}
+
+	/**
+	 * Test that nothing is written to the log file when request debugging is disabled.
+	 */
+	public function test_should_not_write_to_log_file_when_request_debugging_is_disabled() {
+		define( 'AP_DEBUG', true );
+		define( 'AP_DEBUG_TYPES', [ 'response', 'string' ] );
+
+		AspireUpdate\Debug::log_request( 'Test log message.' );
+
+		$this->assertFileDoesNotExist( self::$log_file );
+	}
+
+	/**
+	 * Test that the message is written to the log file.
+	 *
+	 * @dataProvider data_debug_types
+	 *
+	 * @param array $debug_types An array of enabled debug types.
+	 */
+	public function test_should_write_to_log_file( $debug_types ) {
+		define( 'AP_DEBUG', true );
+		define( 'AP_DEBUG_TYPES', $debug_types );
+
+		$message = 'Test log message.';
+
+		AspireUpdate\Debug::log_request( $message );
+
+		$this->assertFileExists(
+			self::$log_file,
+			'The log file was created.'
+		);
+
+		$this->assertStringContainsString(
+			$message,
+			file_get_contents( self::$log_file ),
+			'The message was not logged.'
+		);
+	}
+
+	/**
+	 * Data provider.
+	 *
+	 * @return array[]
+	 */
+	public function data_debug_types() {
+		return [
+			'just "request"'                        => [
+				'debug_types' => [ 'request' ],
+			],
+			'"request" at the start of the array"'  => [
+				'debug_types' => [ 'request', 'response' ],
+			],
+			'"request" in the middle of the array"' => [
+				'debug_types' => [ 'string', 'request', 'response' ],
+			],
+			'"request" at the end of the array"'    => [
+				'debug_types' => [ 'string', 'response', 'request' ],
+			],
+		];
+	}
+}
diff --git a/tests/phpunit/tests/Debug/Debug_LogResponseTest.php b/tests/phpunit/tests/Debug/Debug_LogResponseTest.php
new file mode 100644
index 0000000..8be3605
--- /dev/null
+++ b/tests/phpunit/tests/Debug/Debug_LogResponseTest.php
@@ -0,0 +1,104 @@
+<?php
+/**
+ * Class Debug_LogResponseTest
+ *
+ * @package AspireUpdate
+ */
+
+/**
+ * Tests for Debug::log_response()
+ *
+ * These tests cause constants to be defined.
+ * They must run in separate processes and must not preserve global state.
+ *
+ * @runTestsInSeparateProcesses
+ * @preserveGlobalState disabled
+ *
+ * @covers \AspireUpdate\Debug::log_response
+ */
+class Debug_LogResponseTest extends Debug_UnitTestCase {
+	/**
+	 * Test that nothing is written to the log file when debugging is disabled.
+	 */
+	public function test_should_not_write_to_log_file_when_debugging_is_disabled() {
+		define( 'AP_DEBUG', false );
+		define( 'AP_DEBUG_TYPES', [ 'request', 'response', 'string' ] );
+
+		AspireUpdate\Debug::log_response( 'Test log message.' );
+
+		$this->assertFileDoesNotExist( self::$log_file );
+	}
+
+	/**
+	 * Test that nothing is written to the log file when debug types are not an array.
+	 */
+	public function test_should_not_write_to_log_file_when_debug_types_are_not_an_array() {
+		define( 'AP_DEBUG', true );
+		define( 'AP_DEBUG_TYPES', 'response' );
+
+		AspireUpdate\Debug::log_response( 'Test log message.' );
+
+		$this->assertFileDoesNotExist( self::$log_file );
+	}
+
+	/**
+	 * Test that nothing is written to the log file when response debugging is disabled.
+	 */
+	public function test_should_not_write_to_log_file_when_response_debugging_is_disabled() {
+		define( 'AP_DEBUG', true );
+		define( 'AP_DEBUG_TYPES', [ 'request', 'string' ] );
+
+		AspireUpdate\Debug::log_response( 'Test log message.' );
+
+		$this->assertFileDoesNotExist( self::$log_file );
+	}
+
+	/**
+	 * Test that the message is written to the log file.
+	 *
+	 * @dataProvider data_debug_types
+	 *
+	 * @param array $debug_types An array of enabled debug types.
+	 */
+	public function test_should_write_to_log_file( $debug_types ) {
+		define( 'AP_DEBUG', true );
+		define( 'AP_DEBUG_TYPES', $debug_types );
+
+		$message = 'Test log message.';
+
+		AspireUpdate\Debug::log_response( $message );
+
+		$this->assertFileExists(
+			self::$log_file,
+			'The log file was created.'
+		);
+
+		$this->assertStringContainsString(
+			$message,
+			file_get_contents( self::$log_file ),
+			'The message was not logged.'
+		);
+	}
+
+	/**
+	 * Data provider.
+	 *
+	 * @return array[]
+	 */
+	public function data_debug_types() {
+		return [
+			'just "response"'                        => [
+				'debug_types' => [ 'response' ],
+			],
+			'"response" at the start of the array"'  => [
+				'debug_types' => [ 'response', 'request' ],
+			],
+			'"response" in the middle of the array"' => [
+				'debug_types' => [ 'string', 'response', 'request' ],
+			],
+			'"response" at the end of the array"'    => [
+				'debug_types' => [ 'string', 'request', 'response' ],
+			],
+		];
+	}
+}
diff --git a/tests/phpunit/tests/Debug/Debug_LogStringTest.php b/tests/phpunit/tests/Debug/Debug_LogStringTest.php
new file mode 100644
index 0000000..14508a6
--- /dev/null
+++ b/tests/phpunit/tests/Debug/Debug_LogStringTest.php
@@ -0,0 +1,104 @@
+<?php
+/**
+ * Class Debug_LogStringTest
+ *
+ * @package AspireUpdate
+ */
+
+/**
+ * Tests for Debug::log_string()
+ *
+ * These tests cause constants to be defined.
+ * They must run in separate processes and must not preserve global state.
+ *
+ * @runTestsInSeparateProcesses
+ * @preserveGlobalState disabled
+ *
+ * @covers \AspireUpdate\Debug::log_string
+ */
+class Debug_LogStringTest extends Debug_UnitTestCase {
+	/**
+	 * Test that nothing is written to the log file when debugging is disabled.
+	 */
+	public function test_should_not_write_to_log_file_when_debugging_is_disabled() {
+		define( 'AP_DEBUG', false );
+		define( 'AP_DEBUG_TYPES', [ 'request', 'response', 'string' ] );
+
+		AspireUpdate\Debug::log_string( 'Test log message.' );
+
+		$this->assertFileDoesNotExist( self::$log_file );
+	}
+
+	/**
+	 * Test that nothing is written to the log file when debug types are not an array.
+	 */
+	public function test_should_not_write_to_log_file_when_debug_types_are_not_an_array() {
+		define( 'AP_DEBUG', true );
+		define( 'AP_DEBUG_TYPES', 'string' );
+
+		AspireUpdate\Debug::log_string( 'Test log message.' );
+
+		$this->assertFileDoesNotExist( self::$log_file );
+	}
+
+	/**
+	 * Test that nothing is written to the log file when string debugging is disabled.
+	 */
+	public function test_should_not_write_to_log_file_when_string_debugging_is_disabled() {
+		define( 'AP_DEBUG', true );
+		define( 'AP_DEBUG_TYPES', [ 'request', 'response' ] );
+
+		AspireUpdate\Debug::log_string( 'Test log message.' );
+
+		$this->assertFileDoesNotExist( self::$log_file );
+	}
+
+	/**
+	 * Test that the message is written to the log file.
+	 *
+	 * @dataProvider data_debug_types
+	 *
+	 * @param array $debug_types An array of enabled debug types.
+	 */
+	public function test_should_write_to_log_file( $debug_types ) {
+		define( 'AP_DEBUG', true );
+		define( 'AP_DEBUG_TYPES', $debug_types );
+
+		$message = 'Test log message.';
+
+		AspireUpdate\Debug::log_string( $message );
+
+		$this->assertFileExists(
+			self::$log_file,
+			'The log file was created.'
+		);
+
+		$this->assertStringContainsString(
+			$message,
+			file_get_contents( self::$log_file ),
+			'The message was not logged.'
+		);
+	}
+
+	/**
+	 * Data provider.
+	 *
+	 * @return array[]
+	 */
+	public function data_debug_types() {
+		return [
+			'just "string"'                        => [
+				'debug_types' => [ 'string' ],
+			],
+			'"string" at the start of the array"'  => [
+				'debug_types' => [ 'string', 'request' ],
+			],
+			'"string" in the middle of the array"' => [
+				'debug_types' => [ 'request', 'string', 'response' ],
+			],
+			'"string" at the end of the array"'    => [
+				'debug_types' => [ 'request', 'response', 'string' ],
+			],
+		];
+	}
+}
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' ],
+			],
+		];
+	}
+}
diff --git a/tests/phpunit/tests/Debug/Debug_ReadTest.php b/tests/phpunit/tests/Debug/Debug_ReadTest.php
new file mode 100644
index 0000000..5f275e1
--- /dev/null
+++ b/tests/phpunit/tests/Debug/Debug_ReadTest.php
@@ -0,0 +1,187 @@
+<?php
+/**
+ * Class Debug_ReadTest
+ *
+ * @package AspireUpdate
+ */
+
+/**
+ * Tests for Debug::read()
+ *
+ * @covers \AspireUpdate\Debug::read
+ */
+class Debug_ReadTest extends Debug_UnitTestCase {
+	/**
+	 * Test that a WP_Error object is returned when the filesystem isn't available.
+	 *
+	 * @covers \AspireUpdate\Debug::init_filesystem
+	 * @covers \AspireUpdate\Debug::verify_filesystem
+	 */
+	public function test_should_return_wp_error_when_filesystem_is_not_available() {
+		add_filter( 'filesystem_method', '__return_false' );
+		$this->assertWPError( AspireUpdate\Debug::read() );
+	}
+
+	/**
+	 * Test that a WP_Error object is returned when the log file doesn't exist.
+	 *
+	 * @covers \AspireUpdate\Debug::init_filesystem
+	 * @covers \AspireUpdate\Debug::verify_filesystem
+	 * @covers \AspireUpdate\Debug::get_file_path
+	 */
+	public function test_should_return_wp_error_when_log_file_does_not_exist() {
+		$this->assertWPError( AspireUpdate\Debug::read() );
+	}
+
+	/**
+	 * Test that a WP_Error object is returned when the log file isn't readable.
+	 *
+	 * @covers \AspireUpdate\Debug::init_filesystem
+	 * @covers \AspireUpdate\Debug::verify_filesystem
+	 * @covers \AspireUpdate\Debug::get_file_path
+	 */
+	public function test_should_return_wp_error_when_log_file_is_not_readable() {
+		global $wp_filesystem;
+
+		// Create the log file.
+		file_put_contents( self::$log_file, '' );
+
+		// Backup and replace the filesystem object.
+		$wp_filesystem = $this->get_fake_filesystem( true, false, true );
+
+		$actual = AspireUpdate\Debug::read();
+
+		$this->assertWPError( $actual );
+	}
+
+	/**
+	 * Test that an empty log message is returned when the log file is empty.
+	 */
+	public function test_should_return_an_empty_log_message_when_log_file_is_empty() {
+		file_put_contents( self::$log_file, '' );
+
+		$actual = AspireUpdate\Debug::read();
+		$this->assertIsString(
+			$actual,
+			'A string was not returned.'
+		);
+
+		$this->assertStringContainsString(
+			'Log file is empty',
+			$actual,
+			'The empty log file message was not returned.'
+		);
+	}
+
+	/**
+	 * Test that an empty log message is returned when the log file's content
+	 * is only empty space.
+	 */
+	public function test_should_return_an_empty_log_message_when_log_file_only_has_empty_space() {
+		file_put_contents( self::$log_file, " \n\r\t\v\x00" );
+
+		$actual = AspireUpdate\Debug::read();
+		$this->assertIsString(
+			$actual,
+			'A string was not returned.'
+		);
+
+		$this->assertStringContainsString(
+			'Log file is empty',
+			$actual,
+			'The empty log file message was not returned.'
+		);
+	}
+
+	/**
+	 * Test that an empty log message is not returned when the log file
+	 * has contents.
+	 */
+	public function test_should_not_return_an_empty_log_message_when_log_file_has_contents() {
+		file_put_contents( self::$log_file, 'Some contents' );
+
+		$actual = AspireUpdate\Debug::read();
+		$this->assertIsString(
+			$actual,
+			'A string was not returned.'
+		);
+
+		$this->assertStringNotContainsString(
+			'Log file is empty',
+			$actual,
+			'The empty log file message was returned.'
+		);
+	}
+
+	/**
+	 * Test that a truncation message is added when the log file has more
+	 * lines than requested.
+	 */
+	public function test_should_add_a_truncation_message_when_log_file_has_more_lines_than_requested() {
+		file_put_contents(
+			self::$log_file,
+			"First line\r\nSecond line\r\nThird line"
+		);
+
+		$actual = AspireUpdate\Debug::read( 2 );
+
+		$this->assertIsString(
+			$actual,
+			'A string was not returned.'
+		);
+
+		$this->assertStringContainsString(
+			'Log truncated',
+			$actual,
+			'The truncation message was not returned.'
+		);
+	}
+
+	/**
+	 * Test that no truncation message is added when the log file has the same
+	 * number of lines as requested.
+	 */
+	public function test_should_not_add_a_truncation_message_when_log_file_has_the_same_number_of_lines_as_requested() {
+		file_put_contents(
+			self::$log_file,
+			"First line\r\nSecond line\r\nThird line"
+		);
+
+		$actual = AspireUpdate\Debug::read( 3 );
+
+		$this->assertIsString(
+			$actual,
+			'A string was not returned.'
+		);
+
+		$this->assertStringNotContainsString(
+			'Log truncated',
+			$actual,
+			'The truncation message was added.'
+		);
+	}
+
+	/**
+	 * Test that no truncation message is added when the log file has fewer than
+	 * lines than requested.
+	 */
+	public function test_should_not_add_a_truncation_message_when_log_file_has_fewer_lines_than_requested() {
+		file_put_contents(
+			self::$log_file,
+			"First line\r\nSecond line\r\nThird line"
+		);
+
+		$actual = AspireUpdate\Debug::read( 4 );
+
+		$this->assertIsString(
+			$actual,
+			'A string was not returned.'
+		);
+
+		$this->assertStringNotContainsString(
+			'Log truncated',
+			$actual,
+			'The truncation message was added.'
+		);
+	}
+}