The Close Combat Tactical Trainer (CCTT) is a simulation-based training system being developed using a concurrent engineering approach. Industry and government engineers together with prototypical system users comprise the concurrent engineering teams. User representatives are located, along with industry, in an integrated development facility where they interact on a daily basis with the rest of the engineering team. CCTT is a large-scale complex simulation, its primary purpose is to improve human performance through its use in training. Focusing on human performance improvement as a simulation system product mandates a development effort based on usability engineering. Because CCTT has complex human-computer interaction requirements, field users have been integrated throughout the design, development and evaluation phases of the program.