Read this guide to learn how to use the Application Builder, a set of tools for creating custom applications based on multiphysics models. The Application Builder is available directly in the COMSOL Desktop® and includes a comprehensive set of user interface and programming and debugging tools. See the Introduction to the Application Builder for an overview of the Application Builder and the Application Libraries, where you will find many example apps. See also the Application Programming Guide for information and examples of how to use the programming tools in the Application Builder to create COMSOL apps.