The application developer may need to simulate or prototype his application to identify the major phenomena that may occur, experimenting with a small set of users. Rapid prototyping may be done if an application generator is provided or an adapted application skeleton is available. For complex applications one may use the Wizard of Oz (WOZ) approach that is described in Chapter 13 of this handbook. The application developer should know if such tools are available and how suitable they are for his application.