diff --git a/src/Admin/AbstractOptionsPage.php b/src/Admin/AbstractOptionsPage.php
index 673b216..532700a 100644
--- a/src/Admin/AbstractOptionsPage.php
+++ b/src/Admin/AbstractOptionsPage.php
@@ -95,7 +95,7 @@ public function render(): void {
 	/**
 	 * Configure the admin page using the settings API.
 	 */
-	abstract public function configure();
+	abstract public function configure(): void;
 
 	/**
 	 * Get the capability required to view the admin pages.
@@ -160,7 +160,7 @@ abstract public function enqueue_page_assets( string $hook ): void;
 	 * @param string $page_slug The page slug.
 	 * @param string $script_handle The script handle.
 	 */
-	abstract public function localize_script( string $page_slug, string $script_handle );
+	abstract public function localize_script( string $page_slug, string $script_handle ): void;
 
 	/**
 	 * Add body class.
diff --git a/src/Admin/MetaBox.php b/src/Admin/MetaBox.php
index 8623221..fca1f79 100644
--- a/src/Admin/MetaBox.php
+++ b/src/Admin/MetaBox.php
@@ -45,7 +45,7 @@ public static function register( Options $options ): void {
 	/**
 	 * Add "GTM Kit" meta box
 	 */
-	public function add_meta_boxes() {
+	public function add_meta_boxes(): void {
 		if ( current_user_can( 'manage_options' ) ) {
 			$post_types = get_post_types(
 				[
@@ -77,7 +77,7 @@ public function add_meta_boxes() {
 	/**
 	 * Displays some checkbox to de/activate some cache options
 	 */
-	public function display_meta_boxes() {
+	public function display_meta_boxes(): void {
 		if ( current_user_can( 'manage_options' ) ) {
 			wp_nonce_field( 'gtmkit_box_option', '_gtmkitnonce', false );
 			$page_type = get_post_meta( get_the_ID(), 'gtmkit_page_type', true );
@@ -103,7 +103,7 @@ public function display_meta_boxes() {
 	/**
 	 * Manage the cache options from the meta box.
 	 */
-	public function save_meta_box_options() {
+	public function save_meta_box_options(): void {
 		if ( current_user_can( 'manage_options' ) && isset( $_POST['post_ID'], $_POST['_gtmkitnonce'] ) ) {
 
 			check_admin_referer( 'gtmkit_box_option', '_gtmkitnonce' );
diff --git a/src/Admin/SetupWizard.php b/src/Admin/SetupWizard.php
index 5f4cd73..81edfbe 100644
--- a/src/Admin/SetupWizard.php
+++ b/src/Admin/SetupWizard.php
@@ -75,7 +75,7 @@ public static function get_site_url(): string {
 	/**
 	 * Maybe redirect to the setup wizard after plugin activation.
 	 */
-	public function maybe_redirect_after_activation() {
+	public function maybe_redirect_after_activation(): void {
 
 		if ( wp_doing_ajax() || wp_doing_cron() ) {
 			return;
@@ -118,7 +118,7 @@ public function add_dashboard_page(): void {
 	 *
 	 * @param string $hook The asset hook.
 	 */
-	public function enqueue_assets( string $hook ) {
+	public function enqueue_assets( string $hook ): void {
 
 		if ( strpos( $hook, self::SLUG ) === false ) {
 			return;
@@ -159,7 +159,7 @@ public function enqueue_assets( string $hook ) {
 	/**
 	 * Setup Wizard Content
 	 */
-	public function setup_wizard_content() {
+	public function setup_wizard_content(): void {
 		$admin_url = is_network_admin() ? network_admin_url() : admin_url();
 
 		$this->settings_error_page( 'gtmkit-settings', '<a class="gtmkit-text-color-grey gtmkit-text-sm" href="' . $admin_url . '">' . esc_html__( 'Go back to the Dashboard', 'gtm-kit' ) . '</a>' );
diff --git a/src/Integration/WooCommerce.php b/src/Integration/WooCommerce.php
index c1b9bcb..50fb349 100644
--- a/src/Integration/WooCommerce.php
+++ b/src/Integration/WooCommerce.php
@@ -1066,7 +1066,7 @@ public function Compatibility_With_TI_Wishlist( array $item_data ): array {
 	/**
 	 * Registers the actual data into each endpoint.
 	 */
-	public function extend_store() {
+	public function extend_store(): void {
 
 		// Register into `cart/items`.
 		$this->extend->register_endpoint_data(