This text features three DES software packages that are used to build the computer simulation model. The Arena and Simul8 packages are based on a process flow approach in which passive entities progress through a series of process steps. All three packages incorporate a graphical interface for building the model, which uses a process-based approach (with no requirement for program code to be entered) and also provides an animated display. The combination of a process flow approach and a graphical interface provides a clear and powerful way to build DES models and is widely adopted in many of the DES software packages currently in use.