Abstract. We describe InterPlay, a simulation engine coordinator that supports cooperation and interaction of multiple simulation and execution tools, thus helping to scale-up the design and development cycle of reactive systems. InterPlay involves two main ideas. In the first, we concentrate on the inter-object design approach involving LSCs and the Play-Engine tool, enabling multiple Play-Engines to run in cooperation. This makes possible the distributed design of large-scale systems by different teams, as well as the refinement of parts of a system using different Play-Engines. The second idea concerns combining the inter-object approach with the more conventional intraobject approach, involving, for example, statecharts and Rhapsody. InterPlay makes it possible to run the Play-Engine in cooperation with Rhapsody, and is very useful when some system objects have clear and distinct internal behavior, or in an iterative development process where the design is implementation-oriented...