Every EMBOSS and EMBASSY program has an ACD (AJAX Command Definition) file which describes the application, its options (parameters) and command line interface. The ACD file controls the behaviour of the application at the command line, particularly, all the user input operations. This chapter describes all aspects of ACD file development. It shows how to write application and data definitions, how to perform operations in ACD files and control prompting of the user. Finally, utilities for debugging and testing ACD files are described.