Rather than pages of specific design-to performance descriptions and tolerances, development specifications released at contract award for complex training systems often require the simulation of many aspects of a trainer to be "in accordance with design criteria." The use of this reference enables work to begin on aircrew training devices roughly at the same time as full-scale development of the weapon system being simulated. However, this concurrent approach complicates the determination of the simulation performance required. By properly using a design criteria list, the actual detailed requirements for simulation can be well communicated. This paper explores the use of a design criteria list in a typical weapon system trainer development. Examples of design criteria use and misuse are drawn from on-going simulator programs. The need for weapon system prime contractor involvement, well chosen design criteria freeze dates, and documentation of design assumptions throughout the development cycle is emphasized. Through the process illustrated, the accumulation, distillation, and application of design criteria data is portrayed as the cornerstone of representative simulation of actual weapon system performance. In concurrent weapon system and simulator programs, design criteria may actually be more important than the development specification itself in determining simulation requirements.