The key to facilitating an "adequate" Mission Rehearsal capability lies in the training system itself. The challenge to the military and the simulation industry is to judiciously and cost-effectively apply today's technology to create the simulated aircraft and mission environment fidelity required to meet Mission Rehearsal performance objectives. Traditional development concepts, however, may be inadequate. We are no longer concentrating on training specific tasks, but rather are now involved with complex performance objectives such as tactical awareness, interpretation, anticipation, and prioritization; crew and combined team coordination; and tactical decisionmaking. The criteria for such objectives are not readily definable and in actuality can vary from crew to crew and situation to situation. Consequently, a system to meet such objectives cannot be easily specified. This paper addresses the challenge imposed by Mission Rehearsal performance objectives and fidelity requirements and discusses an effective solution to supplement traditional specifications: Mission-Oriented Simulator Development. This concept, demonstrated on the AH-64 Combat Mission Simulator, provides techniques incorporating military and developer teamwork to assess and refine the evolving system throughout the development cycle, based on the user's evolving "mission" requirements.