This paper presents a formal model for the interface between a simulation and a knowledge-based manager, and discusses the application of this model to practical simulation problems. Models of both the simulation and the knowledge components are presented, and issues of variable selection, time representation and communication costs are addressed. The model is then applied to a case study involving simulation of a small flexible manufacturing system. A knowledge-based scheduling system in the C-Language Production System (CLIPS) is interfaced to the FMS simulation written in SIMSCRIPT. In the context of this case study, a set of heuristics for selecting interface variables and for analyzing and reducing communications overhead are presented and discussed.