Methods, systems, and apparatus, including computer programs encoded on computer storage media, for creating mobile device applications. In one aspect, a method includes determining that a user has entered one or more user inputs into an integrated development environment, to select or arrange a component that defines a portion of an application, and transmitting interpreter code that corresponds to the component to a mobile device, where the interpreter code, when interpreted by a command interpreter on the mobile device, causes the mobile device to generate a mock-up of the portion of the application defined by the component.