Abstract: An interactive graphical tool is provided for designing the user interface of a program-controlled instrument. The tool runs on a computer workstation and is used to model the application code of the instrument as a first network in which sessions of user interaction with the application code are represented by respective elements of the network. The actual user interaction sessions themselves are modelled by respective second networks; each second network includes information for defining the interface states of the modelled user interaction session. The full user interface can thereafter be simulated by progressing through the first model until a user interaction element is met and then entering the corresponding second network; the interface state information contained in the second network is used to drive a simulation of the instrument interfaace on the display of the computer workstation.