This paper describes the Hierarchical Modeling System (HMS). HMS is a tool - a unified and expandable system - which supports the development of analytic and simulator models of computer networks. The same system and workload descriptions can be interpreted as analytic queueing models with optimization techniques or as discrete event simulation models. The rationale behind the development of HMS is that high level analyses incorporating analytic techniques may be used in the early design phase for networks when many options are considered while detailed simulation studies of fewer design alternatives are appropriate during the later stages.