The Engine Maintenance System Evaluation (EnMasse) assesses the effect of different policies, such as centralization, on jet engine intermediate maintenance. This user's guide to EnMasse, a simulation model developed by the authors, describes the processes (module shop, test cell, etc.) in the model. Users can track the engine operation and maintenance process from the flightline through various shops and back. The report delineates essential components of EnMasse that might be employed or modified to model various choices of engine types and maintenance policies.