diff --git a/admin/class-convertkit-mm-admin.php b/admin/class-convertkit-mm-admin.php
index 6d81ac4..d46cd54 100644
--- a/admin/class-convertkit-mm-admin.php
+++ b/admin/class-convertkit-mm-admin.php
@@ -203,8 +203,7 @@ private function check_credentials() {
 				// Remove from settings.
 				$this->settings->delete_credentials();
 
-				// Redirect to General screen, which will now show the ConvertKit_Settings_OAuth screen, because
-				// the Plugin has no access token.
+				// Reload settings screen, to reflect no credentials exist.
 				wp_safe_redirect(
 					add_query_arg(
 						array(
diff --git a/convertkit-membermouse.php b/convertkit-membermouse.php
index 55166f7..24e5c44 100644
--- a/convertkit-membermouse.php
+++ b/convertkit-membermouse.php
@@ -54,6 +54,7 @@
 
 // Load plugin files.
 require CONVERTKIT_MM_PATH . 'includes/class-convertkit-mm-actions.php';
+require CONVERTKIT_MM_PATH . 'includes/class-convertkit-mm-admin-notices.php';
 require CONVERTKIT_MM_PATH . 'includes/class-convertkit-mm-api.php';
 require CONVERTKIT_MM_PATH . 'includes/class-convertkit-mm-resource.php';
 require CONVERTKIT_MM_PATH . 'includes/class-convertkit-mm-resource-custom-fields.php';
diff --git a/includes/class-convertkit-mm-admin-notices.php b/includes/class-convertkit-mm-admin-notices.php
new file mode 100644
index 0000000..59c3f4d
--- /dev/null
+++ b/includes/class-convertkit-mm-admin-notices.php
@@ -0,0 +1,196 @@
+<?php
+/**
+ * ConvertKit MemberMouse Admin Notices class.
+ *
+ * @package    ConvertKit_MM
+ * @author     Kit
+ */
+
+/**
+ * Add and remove persistent error messages across all
+ * WordPress Administration screens.
+ *
+ * @package ConvertKit_MM
+ * @author  Kit
+ */
+class ConvertKit_MM_Admin_Notices {
+
+	/**
+	 * The key prefix to use for stored notices
+	 *
+	 * @since   1.3.7
+	 *
+	 * @var     string
+	 */
+	private $key_prefix = 'convertkit-mm-admin-notices';
+
+	/**
+	 * Register output function to display persistent notices
+	 * in the WordPress Administration, if any exist.
+	 *
+	 * @since   1.3.7
+	 */
+	public function __construct() {
+
+		add_action( 'admin_notices', array( $this, 'output' ) );
+
+	}
+
+	/**
+	 * Output persistent notices in the WordPress Administration
+	 *
+	 * @since   1.3.7
+	 */
+	public function output() {
+
+		// Don't output if we don't have the required capabilities to fix the issue.
+		if ( ! current_user_can( 'manage_options' ) ) {
+			return;
+		}
+
+		// Bail if no notices exist.
+		$notices = get_option( $this->key_prefix );
+		if ( ! $notices ) {
+			return;
+		}
+
+		// Output notices.
+		foreach ( $notices as $notice ) {
+			switch ( $notice ) {
+				case 'authorization_failed':
+					$api    = new ConvertKit_MM_API( CONVERTKIT_MM_OAUTH_CLIENT_ID, CONVERTKIT_MM_OAUTH_CLIENT_REDIRECT_URI );
+					$output = sprintf(
+						'%s %s',
+						esc_html__( 'Kit for MemberMouse: Authorization failed. Please', 'convertkit-mm' ),
+						sprintf(
+							'<a href="%s">%s</a>',
+							esc_url( $api->get_oauth_url( admin_url( 'options-general.php?page=convertkit-mm' ), get_site_url() ) ),
+							esc_html__( 'connect your Kit account.', 'convertkit-mm' )
+						)
+					);
+					break;
+
+				default:
+					$output = '';
+
+					/**
+					 * Define the text to output in an admin error notice.
+					 *
+					 * @since   1.3.7
+					 *
+					 * @param   string  $notice     Admin notice name.
+					 */
+					$output = apply_filters( CONVERTKIT_MM_NAME . '_admin_notices_output_' . $notice, $output );
+					break;
+			}
+
+			// If no output defined, skip.
+			if ( empty( $output ) ) {
+				continue;
+			}
+			?>
+			<div class="notice notice-error">
+				<p>
+					<?php
+					echo wp_kses(
+						$output,
+						wp_kses_allowed_html( 'post' )
+					);
+					?>
+				</p>
+			</div>
+			<?php
+		}
+
+	}
+
+	/**
+	 * Add a persistent notice for output in the WordPress Administration.
+	 *
+	 * @since   1.3.7
+	 *
+	 * @param   string $notice     Notice name.
+	 * @return  bool                Notice saved successfully
+	 */
+	public function add( $notice ) {
+
+		// If no other persistent notices exist, add one now.
+		if ( ! $this->exist() ) {
+			return update_option( $this->key_prefix, array( $notice ) );
+		}
+
+		// Fetch existing persistent notices.
+		$notices = $this->get();
+
+		// Add notice to existing notices.
+		$notices[] = $notice;
+
+		// Remove any duplicate notices.
+		$notices = array_values( array_unique( $notices ) );
+
+		// Update and return.
+		return update_option( $this->key_prefix, $notices );
+
+	}
+
+	/**
+	 * Returns all notices stored in the options table.
+	 *
+	 * @since   1.3.7
+	 *
+	 * @return  array
+	 */
+	public function get() {
+
+		// Fetch all notices from the options table.
+		return get_option( $this->key_prefix );
+
+	}
+
+	/**
+	 * Whether any persistent notices are stored in the option table.
+	 *
+	 * @since   1.3.7
+	 *
+	 * @return  bool
+	 */
+	public function exist() {
+
+		if ( ! $this->get() ) {
+			return false;
+		}
+
+		return true;
+
+	}
+
+	/**
+	 * Delete all persistent notices.
+	 *
+	 * @since   1.3.7
+	 *
+	 * @param   string $notice     Notice name.
+	 * @return  bool                Success
+	 */
+	public function delete( $notice ) {
+
+		// If no persistent notices exist, there's nothing to delete.
+		if ( ! $this->exist() ) {
+			return false;
+		}
+
+		// Fetch existing persistent notices.
+		$notices = $this->get();
+
+		// Remove notice from existing notices.
+		$index = array_search( $notice, $notices, true );
+		if ( $index !== false ) {
+			unset( $notices[ $index ] );
+		}
+
+		// Update and return.
+		return update_option( $this->key_prefix, $notices );
+
+	}
+
+}
diff --git a/includes/class-convertkit-mm-api.php b/includes/class-convertkit-mm-api.php
index a80d00e..510cca9 100644
--- a/includes/class-convertkit-mm-api.php
+++ b/includes/class-convertkit-mm-api.php
@@ -1,15 +1,15 @@
 <?php
 /**
- * ConvertKit API class for WPForms.
+ * ConvertKit API class for MemberMouse.
  *
- * @package ConvertKit_WPForms
+ * @package ConvertKit_MM
  * @author ConvertKit
  */
 
 /**
- * ConvertKit API class for WPForms.
+ * ConvertKit API class for MemberMouse.
  *
- * @package ConvertKit_WPForms
+ * @package ConvertKit_MM
  * @author ConvertKit
  */
 class ConvertKit_MM_API extends ConvertKit_API_V4 {
diff --git a/includes/class-convertkit-mm-resource-tags.php b/includes/class-convertkit-mm-resource-tags.php
index eac0783..9b0843a 100644
--- a/includes/class-convertkit-mm-resource-tags.php
+++ b/includes/class-convertkit-mm-resource-tags.php
@@ -2,7 +2,7 @@
 /**
  * ConvertKit Tags Resource class.
  *
- * @package ConvertKit_WPForms
+ * @package ConvertKit_MM
  * @author ConvertKit
  */
 
diff --git a/includes/class-convertkit-mm-resource.php b/includes/class-convertkit-mm-resource.php
index 13bd530..0899683 100644
--- a/includes/class-convertkit-mm-resource.php
+++ b/includes/class-convertkit-mm-resource.php
@@ -33,8 +33,34 @@ public function __construct() {
 			);
 		}
 
-		// Call parent initialization function.
-		parent::init();
+		// Get last query time and existing resources.
+		$this->last_queried = get_option( $this->settings_name . '_last_queried' );
+		$this->resources    = get_option( $this->settings_name );
+
+	}
+
+	/**
+	 * Fetches resources (custom fields, forms, sequences or tags) from the API, storing them in the options table
+	 * with a last queried timestamp.
+	 *
+	 * If the refresh results in a 401, removes the access and refresh tokens from the settings.
+	 *
+	 * @since   1.3.7
+	 *
+	 * @return  WP_Error|array
+	 */
+	public function refresh() {
+
+		// Call parent refresh method.
+		$result = parent::refresh();
+
+		// If an error occured, maybe delete credentials from the Plugin's settings
+		// if the error is a 401 unauthorized.
+		if ( is_wp_error( $result ) ) {
+			convertkit_mm_maybe_delete_credentials( $result );
+		}
+
+		return $result;
 
 	}
 
diff --git a/includes/class-convertkit-mm-settings.php b/includes/class-convertkit-mm-settings.php
index 3d5ee07..8791d9e 100644
--- a/includes/class-convertkit-mm-settings.php
+++ b/includes/class-convertkit-mm-settings.php
@@ -49,10 +49,6 @@ public function __construct() {
 			$this->settings = array_merge( $this->get_defaults(), $settings );
 		}
 
-		// Update Access Token when refreshed by the API class.
-		add_action( 'convertkit_api_get_access_token', array( $this, 'update_credentials' ), 10, 2 );
-		add_action( 'convertkit_api_refresh_token', array( $this, 'update_credentials' ), 10, 2 );
-
 	}
 
 	/**
@@ -122,6 +118,9 @@ public function has_api_key() {
 	 */
 	public function get_access_token() {
 
+		// Reload settings from options table, to ensure we have the latest tokens.
+		$this->refresh_settings();
+
 		// Return Access Token from settings.
 		return $this->settings['access_token'];
 
@@ -149,6 +148,9 @@ public function has_access_token() {
 	 */
 	public function get_refresh_token() {
 
+		// Reload settings from options table, to ensure we have the latest tokens.
+		$this->refresh_settings();
+
 		// Return Refresh Token from settings.
 		return $this->settings['refresh_token'];
 
@@ -287,16 +289,13 @@ public function get_bundle_cancellation_mapping( $id ) {
 	 *
 	 * @since   1.3.0
 	 *
-	 * @param   array  $result      New Access Token, Refresh Token and Expiry.
-	 * @param   string $client_id   OAuth Client ID used for the Access and Refresh Tokens.
+	 * @param   array $result      New Access Token, Refresh Token and Expiry.
 	 */
-	public function update_credentials( $result, $client_id ) {
+	public function update_credentials( $result ) {
 
-		// Don't save these credentials if they're not for this Client ID.
-		// They're for another ConvertKit Plugin that uses OAuth.
-		if ( $client_id !== CONVERTKIT_MM_OAUTH_CLIENT_ID ) {
-			return;
-		}
+		// Remove any existing persistent notice.
+		$admin_notices = new ConvertKit_MM_Admin_Notices();
+		$admin_notices->delete( 'authorization_failed' );
 
 		$this->save(
 			array(
@@ -329,6 +328,9 @@ public function delete_credentials() {
 			)
 		);
 
+		// Clear any existing scheduled WordPress Cron event.
+		wp_clear_scheduled_hook( 'convertkit_mm_refresh_token' );
+
 	}
 
 	/**
@@ -410,7 +412,25 @@ public function save( $settings ) {
 		update_option( self::SETTINGS_NAME, array_merge( $this->get(), $settings ) );
 
 		// Reload settings in class, to reflect changes.
-		$this->settings = get_option( self::SETTINGS_NAME );
+		$this->refresh_settings();
+
+	}
+
+	/**
+	 * Reloads settings from the options table so this instance has the latest values.
+	 *
+	 * @since  1.3.7
+	 */
+	private function refresh_settings() {
+
+		$settings = get_option( self::SETTINGS_NAME );
+
+		if ( ! $settings ) {
+			$this->settings = $this->get_defaults();
+			return;
+		}
+
+		$this->settings = array_merge( $this->get_defaults(), $settings );
 
 	}
 
diff --git a/includes/class-convertkit-mm.php b/includes/class-convertkit-mm.php
index 382c3ab..a2be708 100644
--- a/includes/class-convertkit-mm.php
+++ b/includes/class-convertkit-mm.php
@@ -55,7 +55,7 @@ class ConvertKit_MM {
 	public function __construct() {
 
 		// Initialize.
-		add_action( 'init', array( $this, 'init' ) );
+		add_action( 'init', array( $this, 'init' ), 1 );
 
 	}
 
@@ -85,7 +85,8 @@ private function initialize_admin() {
 			return;
 		}
 
-		$this->classes['admin'] = new ConvertKit_MM_Admin();
+		$this->classes['admin']         = new ConvertKit_MM_Admin();
+		$this->classes['admin_notices'] = new ConvertKit_MM_Admin_Notices();
 
 		/**
 		 * Initialize integration classes for the WordPress Administration interface.
diff --git a/includes/convertkit-mm-functions.php b/includes/convertkit-mm-functions.php
index 3f3aa31..d2c3830 100644
--- a/includes/convertkit-mm-functions.php
+++ b/includes/convertkit-mm-functions.php
@@ -31,3 +31,68 @@ function convertkit_mm_log( $log, $message ) {
 	fclose( $log ); // phpcs:ignore WordPress.WP.AlternativeFunctions
 
 }
+
+/**
+ * Saves the new access token, refresh token and its expiry, and schedules
+ * a WordPress Cron event to refresh the token on expiry.
+ *
+ * @since   1.3.7
+ *
+ * @param   array  $result      New Access Token, Refresh Token and Expiry.
+ * @param   string $client_id   OAuth Client ID used for the Access and Refresh Tokens.
+ */
+function convertkit_mm_maybe_update_credentials( $result, $client_id ) {
+
+	// Don't save these credentials if they're not for this Client ID.
+	// They're for another Kit Plugin that uses OAuth.
+	if ( $client_id !== CONVERTKIT_MM_OAUTH_CLIENT_ID ) {
+		return;
+	}
+
+	$settings = new ConvertKit_MM_Settings();
+	$settings->update_credentials( $result );
+
+}
+
+/**
+ * Deletes the stored access token, refresh token and its expiry from the Plugin settings,
+ * and clears any existing scheduled WordPress Cron event to refresh the token on expiry,
+ * when either:
+ * - The access token is invalid
+ * - The access token expired, and refreshing failed
+ *
+ * @since   1.3.7
+ *
+ * @param   WP_Error $result      Error result.
+ * @param   string   $client_id   OAuth Client ID used for the Access and Refresh Tokens.
+ */
+function convertkit_mm_maybe_delete_credentials( $result, $client_id ) {
+
+	// Don't save these credentials if they're not for this Client ID.
+	// They're for another Kit Plugin that uses OAuth.
+	if ( $client_id !== CONVERTKIT_MM_OAUTH_CLIENT_ID ) {
+		return;
+	}
+
+	// If the error isn't a 401, don't delete credentials.
+	// This could be e.g. a temporary network error, rate limit or similar.
+	if ( $result->get_error_data( 'convertkit_api_error' ) !== 401 ) {
+		return;
+	}
+
+	// Persist an error notice in the WordPress Administration until the user fixes the problem.
+	$admin_notices = new ConvertKit_MM_Admin_Notices();
+	$admin_notices->add( 'authorization_failed' );
+
+	$settings = new ConvertKit_MM_Settings();
+	$settings->delete_credentials();
+
+}
+
+// Update Access Token when refreshed by the API class.
+add_action( 'convertkit_api_get_access_token', 'convertkit_mm_maybe_update_credentials', 10, 2 );
+add_action( 'convertkit_api_refresh_token', 'convertkit_mm_maybe_update_credentials', 10, 2 );
+
+// Delete credentials if the API class uses a invalid access token.
+// This prevents the Plugin making repetitive API requests that will 401.
+add_action( 'convertkit_api_access_token_invalid', 'convertkit_mm_maybe_delete_credentials', 10, 2 );
diff --git a/tests/EndToEnd/general/SettingsCest.php b/tests/EndToEnd/general/SettingsCest.php
index e344e98..f73c22c 100644
--- a/tests/EndToEnd/general/SettingsCest.php
+++ b/tests/EndToEnd/general/SettingsCest.php
@@ -90,6 +90,9 @@ public function testInvalidCredentials(EndToEndTester $I)
 		$I->see('Connect');
 		$I->dontSee('Disconnect');
 		$I->dontSeeElementInDOM('input#submit');
+
+		// Check that a notice is displayed that the API credentials are invalid.
+		$I->seeErrorNotice($I, 'Kit for MemberMouse: Authorization failed. Please connect your Kit account.');
 	}
 
 	/**
@@ -122,6 +125,15 @@ public function testValidCredentials(EndToEndTester $I)
 		$I->see('Disconnect');
 		$I->seeElementInDOM('input#submit');
 
+		// Navigate to the WordPress Admin.
+		$I->amOnAdminPage('index.php');
+
+		// Check that no notice is displayed that the API credentials are invalid.
+		$I->dontSeeErrorNotice($I, 'Kit for MemberMouse: Authorization failed. Please connect your Kit account.');
+
+		// Go to the Plugin's Settings Screen.
+		$I->amOnAdminPage('options-general.php?page=convertkit-mm');
+
 		// Disconnect the Plugin connection to ConvertKit.
 		$I->click('Disconnect');
 
diff --git a/tests/Integration/ResourceCustomFieldsNoDataTest.php b/tests/Integration/ResourceCustomFieldsNoDataTest.php
new file mode 100644
index 0000000..112e253
--- /dev/null
+++ b/tests/Integration/ResourceCustomFieldsNoDataTest.php
@@ -0,0 +1,162 @@
+<?php
+
+namespace Tests;
+
+use lucatume\WPBrowser\TestCase\WPTestCase;
+
+/**
+ * Tests for the CKWC_Resource_Custom_Fields class when no data is present in the API.
+ *
+ * @since   1.3.7
+ */
+class ResourceCustomFieldsNoDataTest extends \Codeception\TestCase\WPTestCase
+{
+	/**
+	 * The testing implementation.
+	 *
+	 * @var \WpunitTester.
+	 */
+	protected $tester;
+
+	/**
+	 * Holds the ConvertKit Settings class.
+	 *
+	 * @since   1.3.7
+	 *
+	 * @var     ConvertKit_MM_Settings
+	 */
+	private $settings;
+
+	/**
+	 * Holds the ConvertKit Resource class.
+	 *
+	 * @since   1.3.7
+	 *
+	 * @var     ConvertKit_MM_Resource_Custom_Fields
+	 */
+	private $resource;
+
+	/**
+	 * Performs actions before each test.
+	 *
+	 * @since   1.3.7
+	 */
+	public function setUp(): void
+	{
+		parent::setUp();
+
+		// Activate Plugin.
+		activate_plugins('convertkit/wp-convertkit.php');
+
+		// Store credentials in Plugin's settings.
+		$this->settings = new \ConvertKit_MM_Settings();
+		update_option(
+			$this->settings::SETTINGS_NAME,
+			[
+				'access_token'  => $_ENV['CONVERTKIT_OAUTH_ACCESS_TOKEN_NO_DATA'],
+				'refresh_token' => $_ENV['CONVERTKIT_OAUTH_REFRESH_TOKEN_NO_DATA'],
+			]
+		);
+
+		// Initialize the resource class we want to test.
+		$this->resource = new \ConvertKit_MM_Resource_Custom_Fields();
+
+		// Confirm initialization didn't result in an error.
+		$this->assertNotInstanceOf(\WP_Error::class, $this->resource->resources);
+
+		// Initialize the resource class, fetching resources from the API and caching them in the options table.
+		$result = $this->resource->init();
+
+		// Confirm calling init() didn't result in an error.
+		$this->assertNotInstanceOf(\WP_Error::class, $result);
+	}
+
+	/**
+	 * Performs actions after each test.
+	 *
+	 * @since   1.3.7
+	 */
+	public function tearDown(): void
+	{
+		// Delete Credentials and Resources from Plugin's settings.
+		delete_option($this->settings::SETTINGS_NAME);
+		delete_option($this->resource->settings_name);
+		delete_option($this->resource->settings_name . '_last_queried');
+
+		// Destroy the resource class we tested.
+		unset($this->resource);
+
+		// Deactivate Plugin.
+		deactivate_plugins('convertkit/wp-convertkit.php');
+
+		parent::tearDown();
+	}
+
+	/**
+	 * Test that the refresh() function performs as expected.
+	 *
+	 * @since   1.3.7
+	 */
+	public function testRefresh()
+	{
+		// Confirm that no resources exist in the stored options table data.
+		$result = $this->resource->refresh();
+		$this->assertNotInstanceOf(\WP_Error::class, $result);
+		$this->assertIsArray($result);
+		$this->assertCount(0, $result);
+	}
+
+	/**
+	 * Test that the expiry timestamp is set and returns the expected value.
+	 *
+	 * @since   1.3.7
+	 */
+	public function testExpiry()
+	{
+		// Define the expected expiry date based on the resource class' $cache_duration setting.
+		$expectedExpiryDate = date('Y-m-d', time() + $this->resource->cache_duration);
+
+		// Fetch the actual expiry date set when the resource class was initialized.
+		$expiryDate = date('Y-m-d', $this->resource->last_queried + $this->resource->cache_duration);
+
+		// Confirm both dates match.
+		$this->assertEquals($expectedExpiryDate, $expiryDate);
+	}
+
+	/**
+	 * Test that the get() function performs as expected.
+	 *
+	 * @since   1.3.7
+	 */
+	public function testGet()
+	{
+		// Confirm that no resources exist in the stored options table data.
+		$result = $this->resource->get();
+		$this->assertNotInstanceOf(\WP_Error::class, $result);
+		$this->assertIsArray($result);
+		$this->assertCount(0, $result);
+	}
+
+	/**
+	 * Test that the count() function returns the number of resources.
+	 *
+	 * @since   1.3.7
+	 */
+	public function testCount()
+	{
+		$result = $this->resource->get();
+		$this->assertEquals($this->resource->count(), count($result));
+	}
+
+	/**
+	 * Test that the exist() function performs as expected.
+	 *
+	 * @since   1.3.7
+	 */
+	public function testExist()
+	{
+		// Confirm that the function returns false, because resources do not exist.
+		$result = $this->resource->exist();
+		$this->assertSame($result, false);
+	}
+}
diff --git a/tests/Integration/ResourceCustomFieldsTest.php b/tests/Integration/ResourceCustomFieldsTest.php
new file mode 100644
index 0000000..db22ec0
--- /dev/null
+++ b/tests/Integration/ResourceCustomFieldsTest.php
@@ -0,0 +1,242 @@
+<?php
+
+namespace Tests;
+
+use lucatume\WPBrowser\TestCase\WPTestCase;
+
+/**
+ * Tests for the ConvertKit_MM_Resource_Custom_Fields class.
+ *
+ * @since   1.3.7
+ */
+class ResourceCustomFieldsTest extends WPTestCase
+{
+	/**
+	 * The testing implementation.
+	 *
+	 * @var \WpunitTester.
+	 */
+	protected $tester;
+
+	/**
+	 * Holds the ConvertKit Settings class.
+	 *
+	 * @since   1.3.7
+	 *
+	 * @var     ConvertKit_MM_Settings
+	 */
+	private $settings;
+
+	/**
+	 * Holds the ConvertKit Resource class.
+	 *
+	 * @since   1.3.7
+	 *
+	 * @var     ConvertKit_MM_Resource_Custom_Fields
+	 */
+	private $resource;
+
+	/**
+	 * Performs actions before each test.
+	 *
+	 * @since   1.3.7
+	 */
+	public function setUp(): void
+	{
+		parent::setUp();
+
+		// Activate Plugin.
+		activate_plugins('convertkit/wp-convertkit.php');
+
+		// Store Credentials in Plugin's settings.
+		$this->settings = new \ConvertKit_MM_Settings();
+		update_option(
+			$this->settings::SETTINGS_NAME,
+			[
+				'access_token'  => $_ENV['CONVERTKIT_OAUTH_ACCESS_TOKEN'],
+				'refresh_token' => $_ENV['CONVERTKIT_OAUTH_REFRESH_TOKEN'],
+			]
+		);
+
+		// Initialize the resource class we want to test.
+		$this->resource = new \ConvertKit_MM_Resource_Custom_Fields();
+
+		// Confirm initialization didn't result in an error.
+		$this->assertNotInstanceOf(\WP_Error::class, $this->resource->resources);
+
+		// Initialize the resource class, fetching resources from the API and caching them in the options table.
+		$result = $this->resource->init();
+
+		// Confirm calling init() didn't result in an error.
+		$this->assertNotInstanceOf(\WP_Error::class, $result);
+	}
+
+	/**
+	 * Performs actions after each test.
+	 *
+	 * @since   1.3.7
+	 */
+	public function tearDown(): void
+	{
+		// Delete Credentials and Resources from Plugin's settings.
+		delete_option($this->settings::SETTINGS_NAME);
+		delete_option($this->resource->settings_name);
+		delete_option($this->resource->settings_name . '_last_queried');
+
+		// Destroy the resource class we tested.
+		unset($this->resource);
+
+		// Deactivate Plugin.
+		deactivate_plugins('convertkit/wp-convertkit.php');
+
+		parent::tearDown();
+	}
+
+	/**
+	 * Test that the refresh() function performs as expected.
+	 *
+	 * @since   1.3.7
+	 */
+	public function testRefresh()
+	{
+		// Confirm that the data is stored in the options table and includes some expected keys.
+		$result = $this->resource->refresh();
+		$this->assertIsArray($result);
+		$this->assertArrayHasKey('id', reset($result));
+		$this->assertArrayHasKey('name', reset($result));
+	}
+
+	/**
+	 * Test that the expiry timestamp is set and returns the expected value.
+	 *
+	 * @since   1.3.7
+	 */
+	public function testExpiry()
+	{
+		// Define the expected expiry date based on the resource class' $cache_duration setting.
+		$expectedExpiryDate = date('Y-m-d', time() + $this->resource->cache_duration);
+
+		// Fetch the actual expiry date set when the resource class was initialized.
+		$expiryDate = date('Y-m-d', $this->resource->last_queried + $this->resource->cache_duration);
+
+		// Confirm both dates match.
+		$this->assertEquals($expectedExpiryDate, $expiryDate);
+	}
+
+	/**
+	 * Tests that the get() function returns resources in alphabetical ascending order
+	 * by default.
+	 *
+	 * @since   1.3.7
+	 */
+	public function testGet()
+	{
+		// Call resource class' get() function.
+		$result = $this->resource->get();
+
+		// Assert result is an array.
+		$this->assertIsArray($result);
+
+		// Assert top level array keys are preserved.
+		$this->assertArrayHasKey(array_key_first($this->resource->resources), $result);
+		$this->assertArrayHasKey(array_key_last($this->resource->resources), $result);
+
+		// Assert resource within results has expected array keys.
+		$this->assertArrayHasKey('id', reset($result));
+		$this->assertArrayHasKey('name', reset($result));
+		$this->assertArrayHasKey('key', reset($result));
+		$this->assertArrayHasKey('label', reset($result));
+
+		// Assert order of data is in ascending alphabetical order.
+		$this->assertEquals('Billing Address', reset($result)[ $this->resource->order_by ]);
+		$this->assertEquals('URL', end($result)[ $this->resource->order_by ]);
+	}
+
+	/**
+	 * Tests that the get() function returns resources in alphabetical descending order
+	 * when a valid order_by and order properties are defined.
+	 *
+	 * @since   1.3.7
+	 */
+	public function testGetWithValidOrderByAndOrder()
+	{
+		// Define order_by and order.
+		$this->resource->order_by = 'key';
+		$this->resource->order    = 'desc';
+
+		// Call resource class' get() function.
+		$result = $this->resource->get();
+
+		// Assert result is an array.
+		$this->assertIsArray($result);
+
+		// Assert top level array keys are preserved.
+		$this->assertArrayHasKey(array_key_first($this->resource->resources), $result);
+		$this->assertArrayHasKey(array_key_last($this->resource->resources), $result);
+
+		// Assert resource within results has expected array keys.
+		$this->assertArrayHasKey('id', reset($result));
+		$this->assertArrayHasKey('name', reset($result));
+		$this->assertArrayHasKey('key', reset($result));
+		$this->assertArrayHasKey('label', reset($result));
+
+		// Assert order of data is in descending alphabetical order.
+		$this->assertEquals('url', reset($result)[ $this->resource->order_by ]);
+		$this->assertEquals('billing_address', end($result)[ $this->resource->order_by ]);
+	}
+
+	/**
+	 * Tests that the get() function returns resources in their original order
+	 * when populated with Forms and an invalid order_by value is specified.
+	 *
+	 * @since   1.3.7
+	 */
+	public function testGetWithInvalidOrderBy()
+	{
+		// Define order_by with an invalid value (i.e. an array key that does not exist).
+		$this->resource->order_by = 'invalid_key';
+
+		// Call resource class' get() function.
+		$result = $this->resource->get();
+
+		// Assert result is an array.
+		$this->assertIsArray($result);
+
+		// Assert top level array keys are preserved.
+		$this->assertArrayHasKey(array_key_first($this->resource->resources), $result);
+		$this->assertArrayHasKey(array_key_last($this->resource->resources), $result);
+
+		// Assert resource within results has expected array keys.
+		$this->assertArrayHasKey('id', reset($result));
+		$this->assertArrayHasKey('name', reset($result));
+		$this->assertArrayHasKey('key', reset($result));
+		$this->assertArrayHasKey('label', reset($result));
+
+		// Assert order of data has not changed.
+		$this->assertEquals('URL', reset($result)['label']);
+		$this->assertEquals('Notes', end($result)['label']);
+	}
+
+	/**
+	 * Test that the count() function returns the number of resources.
+	 *
+	 * @since   1.3.7
+	 */
+	public function testCount()
+	{
+		$result = $this->resource->get();
+		$this->assertEquals($this->resource->count(), count($result));
+	}
+
+	/**
+	 * Test that the exist() function performs as expected.
+	 *
+	 * @since   1.3.7
+	 */
+	public function testExist()
+	{
+		// Confirm that the function returns true, because resources exist.
+		$result = $this->resource->exist();
+		$this->assertSame($result, true);
+	}
+}
diff --git a/tests/Integration/ResourceTagsNoDataTest.php b/tests/Integration/ResourceTagsNoDataTest.php
new file mode 100644
index 0000000..7ded369
--- /dev/null
+++ b/tests/Integration/ResourceTagsNoDataTest.php
@@ -0,0 +1,162 @@
+<?php
+
+namespace Tests;
+
+use lucatume\WPBrowser\TestCase\WPTestCase;
+
+/**
+ * Tests for the ConvertKit_Resource_Tags class when no data is present in the API.
+ *
+ * @since   1.3.7
+ */
+class ResourceTagsNoDataTest extends WPTestCase
+{
+	/**
+	 * The testing implementation.
+	 *
+	 * @var \WpunitTester.
+	 */
+	protected $tester;
+
+	/**
+	 * Holds the ConvertKit Settings class.
+	 *
+	 * @since   1.3.7
+	 *
+	 * @var     ConvertKit_MM_Settings
+	 */
+	private $settings;
+
+	/**
+	 * Holds the ConvertKit Resource class.
+	 *
+	 * @since   1.3.7
+	 *
+	 * @var     ConvertKit_MM_Resource_Tags
+	 */
+	private $resource;
+
+	/**
+	 * Performs actions before each test.
+	 *
+	 * @since   1.3.7
+	 */
+	public function setUp(): void
+	{
+		parent::setUp();
+
+		// Activate Plugin.
+		activate_plugins('convertkit/wp-convertkit.php');
+
+		// Store credentials in Plugin's settings.
+		$this->settings = new \ConvertKit_MM_Settings();
+		update_option(
+			$this->settings::SETTINGS_NAME,
+			[
+				'access_token'  => $_ENV['CONVERTKIT_OAUTH_ACCESS_TOKEN_NO_DATA'],
+				'refresh_token' => $_ENV['CONVERTKIT_OAUTH_REFRESH_TOKEN_NO_DATA'],
+			]
+		);
+
+		// Initialize the resource class we want to test.
+		$this->resource = new \ConvertKit_MM_Resource_Tags();
+
+		// Confirm initialization didn't result in an error.
+		$this->assertNotInstanceOf(\WP_Error::class, $this->resource->resources);
+
+		// Initialize the resource class, fetching resources from the API and caching them in the options table.
+		$result = $this->resource->init();
+
+		// Confirm calling init() didn't result in an error.
+		$this->assertNotInstanceOf(\WP_Error::class, $result);
+	}
+
+	/**
+	 * Performs actions after each test.
+	 *
+	 * @since   1.3.7
+	 */
+	public function tearDown(): void
+	{
+		// Delete Credentials and Resources from Plugin's settings.
+		delete_option($this->settings::SETTINGS_NAME);
+		delete_option($this->resource->settings_name);
+		delete_option($this->resource->settings_name . '_last_queried');
+
+		// Destroy the resource class we tested.
+		unset($this->resource);
+
+		// Deactivate Plugin.
+		deactivate_plugins('convertkit/wp-convertkit.php');
+
+		parent::tearDown();
+	}
+
+	/**
+	 * Test that the refresh() function performs as expected.
+	 *
+	 * @since   1.3.7
+	 */
+	public function testRefresh()
+	{
+		// Confirm that the data is stored in the options table and includes some expected keys.
+		$result = $this->resource->refresh();
+		$this->assertNotInstanceOf(\WP_Error::class, $result);
+		$this->assertIsArray($result);
+		$this->assertCount(0, $result);
+	}
+
+	/**
+	 * Test that the expiry timestamp is set and returns the expected value.
+	 *
+	 * @since   1.3.7
+	 */
+	public function testExpiry()
+	{
+		// Define the expected expiry date based on the resource class' $cache_duration setting.
+		$expectedExpiryDate = date('Y-m-d', time() + $this->resource->cache_duration);
+
+		// Fetch the actual expiry date set when the resource class was initialized.
+		$expiryDate = date('Y-m-d', $this->resource->last_queried + $this->resource->cache_duration);
+
+		// Confirm both dates match.
+		$this->assertEquals($expectedExpiryDate, $expiryDate);
+	}
+
+	/**
+	 * Test that the get() function performs as expected.
+	 *
+	 * @since   1.3.7
+	 */
+	public function testGet()
+	{
+		// Confirm that the data is fetched from the options table when using get(), and includes some expected keys.
+		$result = $this->resource->get();
+		$this->assertNotInstanceOf(\WP_Error::class, $result);
+		$this->assertIsArray($result);
+		$this->assertCount(0, $result);
+	}
+
+	/**
+	 * Test that the count() function returns the number of resources.
+	 *
+	 * @since   1.3.7
+	 */
+	public function testCount()
+	{
+		$result = $this->resource->get();
+		$this->assertEquals($this->resource->count(), count($result));
+	}
+
+	/**
+	 * Test that the exist() function performs as expected.
+	 *
+	 * @since   1.3.7
+	 */
+	public function testExist()
+	{
+		// Confirm that the function returns true, because resources exist.
+		$result = $this->resource->exist();
+		$this->assertSame($result, false);
+	}
+}
diff --git a/tests/Integration/ResourceTagsTest.php b/tests/Integration/ResourceTagsTest.php
new file mode 100644
index 0000000..a5a0806
--- /dev/null
+++ b/tests/Integration/ResourceTagsTest.php
@@ -0,0 +1,236 @@
+<?php
+
+namespace Tests;
+
+use lucatume\WPBrowser\TestCase\WPTestCase;
+
+/**
+ * Tests for the ConvertKit_Resource_Tags class.
+ *
+ * @since   1.3.7
+ */
+class ResourceTagsTest extends WPTestCase
+{
+	/**
+	 * The testing implementation.
+	 *
+	 * @var \WpunitTester.
+	 */
+	protected $tester;
+
+	/**
+	 * Holds the ConvertKit Settings class.
+	 *
+	 * @since   1.3.7
+	 *
+	 * @var     ConvertKit_MM_Settings
+	 */
+	private $settings;
+
+	/**
+	 * Holds the ConvertKit Resource class.
+	 *
+	 * @since   1.3.7
+	 *
+	 * @var     ConvertKit_MM_Resource_Tags
+	 */
+	private $resource;
+
+	/**
+	 * Performs actions before each test.
+	 *
+	 * @since   1.3.7
+	 */
+	public function setUp(): void
+	{
+		parent::setUp();
+
+		// Activate Plugin.
+		activate_plugins('convertkit/wp-convertkit.php');
+
+		// Store Credentials in Plugin's settings.
+		$this->settings = new \ConvertKit_MM_Settings();
+		update_option(
+			$this->settings::SETTINGS_NAME,
+			[
+				'access_token'  => $_ENV['CONVERTKIT_OAUTH_ACCESS_TOKEN'],
+				'refresh_token' => $_ENV['CONVERTKIT_OAUTH_REFRESH_TOKEN'],
+			]
+		);
+
+		// Initialize the resource class we want to test.
+		$this->resource = new \ConvertKit_MM_Resource_Tags();
+
+		// Confirm initialization didn't result in an error.
+		$this->assertNotInstanceOf(\WP_Error::class, $this->resource->resources);
+
+		// Initialize the resource class, fetching resources from the API and caching them in the options table.
+		$result = $this->resource->init();
+
+		// Confirm calling init() didn't result in an error.
+		$this->assertNotInstanceOf(\WP_Error::class, $result);
+	}
+
+	/**
+	 * Performs actions after each test.
+	 *
+	 * @since   1.3.7
+	 */
+	public function tearDown(): void
+	{
+		// Delete Credentials and Resources from Plugin's settings.
+		delete_option($this->settings::SETTINGS_NAME);
+		delete_option($this->resource->settings_name);
+		delete_option($this->resource->settings_name . '_last_queried');
+
+		// Destroy the resource class we tested.
+		unset($this->resource);
+
+		// Deactivate Plugin.
+		deactivate_plugins('convertkit/wp-convertkit.php');
+
+		parent::tearDown();
+	}
+
+	/**
+	 * Test that the refresh() function performs as expected.
+	 *
+	 * @since   1.3.7
+	 */
+	public function testRefresh()
+	{
+		// Confirm that the data is stored in the options table and includes some expected keys.
+		$result = $this->resource->refresh();
+		$this->assertIsArray($result);
+		$this->assertArrayHasKey('id', reset($result));
+		$this->assertArrayHasKey('name', reset($result));
+	}
+
+	/**
+	 * Test that the expiry timestamp is set and returns the expected value.
+	 *
+	 * @since   1.3.7
+	 */
+	public function testExpiry()
+	{
+		// Define the expected expiry date based on the resource class' $cache_duration setting.
+		$expectedExpiryDate = date('Y-m-d', time() + $this->resource->cache_duration);
+
+		// Fetch the actual expiry date set when the resource class was initialized.
+		$expiryDate = date('Y-m-d', $this->resource->last_queried + $this->resource->cache_duration);
+
+		// Confirm both dates match.
+		$this->assertEquals($expectedExpiryDate, $expiryDate);
+	}
+
+	/**
+	 * Tests that the get() function returns resources in alphabetical ascending order
+	 * by default.
+	 *
+	 * @since   1.3.7
+	 */
+	public function testGet()
+	{
+		// Call resource class' get() function.
+		$result = $this->resource->get();
+
+		// Assert result is an array.
+		$this->assertIsArray($result);
+
+		// Assert top level array keys are preserved.
+		$this->assertArrayHasKey(array_key_first($this->resource->resources), $result);
+		$this->assertArrayHasKey(array_key_last($this->resource->resources), $result);
+
+		// Assert resource within results has expected array keys.
+		$this->assertArrayHasKey('id', reset($result));
+		$this->assertArrayHasKey('name', reset($result));
+
+		// Assert order of data is in ascending alphabetical order.
+		$this->assertEquals('gravityforms-tag-1', reset($result)[ $this->resource->order_by ]);
+		$this->assertEquals('wpforms', end($result)[ $this->resource->order_by ]);
+	}
+
+	/**
+	 * Tests that the get() function returns resources in alphabetical descending order
+	 * when a valid order_by and order properties are defined.
+	 *
+	 * @since   1.3.7
+	 */
+	public function testGetWithValidOrderByAndOrder()
+	{
+		// Define order_by and order.
+		$this->resource->order_by = 'name';
+		$this->resource->order    = 'desc';
+
+		// Call resource class' get() function.
+		$result = $this->resource->get();
+
+		// Assert result is an array.
+		$this->assertIsArray($result);
+
+		// Assert top level array keys are preserved.
+		$this->assertArrayHasKey(array_key_first($this->resource->resources), $result);
+		$this->assertArrayHasKey(array_key_last($this->resource->resources), $result);
+
+		// Assert resource within results has expected array keys.
+		$this->assertArrayHasKey('id', reset($result));
+		$this->assertArrayHasKey('name', reset($result));
+
+		// Assert order of data is in ascending alphabetical order.
+		$this->assertEquals('wpforms', reset($result)[ $this->resource->order_by ]);
+		$this->assertEquals('gravityforms-tag-1', end($result)[ $this->resource->order_by ]);
+	}
+
+	/**
+	 * Tests that the get() function returns resources in their original order
+	 * when populated with Forms and an invalid order_by value is specified.
+	 *
+	 * @since   1.3.7
+	 */
+	public function testGetWithInvalidOrderBy()
+	{
+		// Define order_by with an invalid value (i.e. an array key that does not exist).
+		$this->resource->order_by = 'invalid_key';
+
+		// Call resource class' get() function.
+		$result = $this->resource->get();
+
+		// Assert result is an array.
+		$this->assertIsArray($result);
+
+		// Assert top level array keys are preserved.
+		$this->assertArrayHasKey(array_key_first($this->resource->resources), $result);
+		$this->assertArrayHasKey(array_key_last($this->resource->resources), $result);
+
+		// Assert resource within results has expected array keys.
+		$this->assertArrayHasKey('id', reset($result));
+		$this->assertArrayHasKey('name', reset($result));
+
+		// Assert order of data has not changed.
+		$this->assertEquals('wpforms', reset($result)['name']);
+		$this->assertEquals('wordpress', end($result)['name']);
+	}
+
+	/**
+	 * Test that the count() function returns the number of resources.
+	 *
+	 * @since   1.3.7
+	 */
+	public function testCount()
+	{
+		$result = $this->resource->get();
+		$this->assertEquals($this->resource->count(), count($result));
+	}
+
+	/**
+	 * Test that the exist() function performs as expected.
+	 *
+	 * @since   1.3.7
+	 */
+	public function testExist()
+	{
+		// Confirm that the function returns true, because resources exist.
+		$result = $this->resource->exist();
+		$this->assertSame($result, true);
+	}
+}
diff --git a/tests/Support/Helper/Plugin.php b/tests/Support/Helper/Plugin.php
index 6787fe3..1ccdc31 100644
--- a/tests/Support/Helper/Plugin.php
+++ b/tests/Support/Helper/Plugin.php
@@ -83,5 +83,14 @@ public function resetConvertKitPlugin($I)
 	{
 		// Plugin Settings.
 		$I->dontHaveOptionInDatabase('convertkit-mm-options');
+
+		// Resources.
+		$I->dontHaveOptionInDatabase('convertkit-mm-custom-fields');
+		$I->dontHaveOptionInDatabase('convertkit-mm-custom-fields_last_queried');
+		$I->dontHaveOptionInDatabase('convertkit-mm-tags');
+		$I->dontHaveOptionInDatabase('convertkit-mm-tags_last_queried');
+
+		// Persistent notices.
+		$I->dontHaveOptionInDatabase('convertkit-mm-admin-notices');
 	}
 }