In the past decade, tremendous advances in training concepts and training technologies have occurred. These advances have raised the available training capabilities to levels well beyond the state of the art of ten years ago. The challenge now is to integrate these varied systems to permit combat forces to rehearse collectively in a real-time environment. Current literature provides an increasing set of requirements necessary to provide proper team interactions during mission rehearsal, and these requirements consistently point toward training system interoperability. Interoperability, however, is not clearly defined. The networking of existing and newly-procured training systems, with the intent of rehearsing a specific mission, requires that many facets of the environment be common to all team members. Real-time events, communications, weapon effects, threat, weather, and terrain portrayals are a sampling of these facets. Additionally, the subject of controlling a set of interoperable training systems must be addressed. This paper discusses interoperability between training systems and how such systems can be used to improve the scope of successful training currently available. It then addresses the requirements for interoperable training systems and proposes a set of design requirements which are necessary to provide training system interoperability.