The design of satellite and deep space communications systems frequently involves problems that cannot be solved through pure analysis. In these cases simulation is an invaluable system design tool. This paper describes a flexible communication system simulation package, SYSTID, which has been under continual development and use at Hughes Space and Communications Group for over 15 years. We discuss the history of SYSTID's development as well as its internal workings. Both the software structure and analytic techniques are described. The paper includes several examples, one of which demonstrates SYSTID's topological input language. Hardware comparisons which show the accuracy and limitations of the simulation approach are also presented.