The REST API enables powerful integrations and modern JavaScript applications. Register custom endpoints using register_rest_route in the rest_api_init hook. Define route patterns, methods, and permission callbacks. Use WP_REST_Request to access parameters and WP_REST_Response for returns. Implement proper authentication and capability checks. Custom post types automatically get REST endpoints with show_in_rest. Add custom fields to responses using register_rest_field. Handle errors appropriately with WP_Error objects. The REST API opens WordPress to headless architectures and mobile apps. Understanding REST principles enhances your plugin’s integration possibilities.