This tutorial introduces the MODSIM III language, showing how its simulation "world view" together with its object-oriented architecture and built in graphics contribute to successful simulation model building. 1 WHAT IS MODSIM III? Discrete systems simulation is one of the few truly new capabilities offered by the computing revolution. Computer simulation models provides us with a fascinating means to develop insight into the behaviors of the complex non-deterministic systems which surround us - in communications networks, transportation logistics, business and manufacturing processes, to name a few. These systems are costly to develop and modify. Feasibility analysis and performance prediction through simulation can greatly reduce the risk of failure or wasteful expense. Simulation is an exploratory technique. We develop a model which represents our best understanding of a proposed system or modification. We run the model in the hope that it will confirm our design intuiti...