This report details the design of the Seamless Model Integration program (SEMINT), a software connection system that connects engineering and simulation models without significant reprogramming. The authors used SEMINT to augment the JANUS combat simulation with improved target acquisition models, surface-to-air combat models, and flight planners. The approach is less expensive than combining these models into a single, monolithic entity. Since recoding is not particularly intrusive, the models can still run in their original stand-alone form. The advantage of our approach is the reuse of existing models, simultaneous use of multiple processors, use of otherwise incompatible programming languages and systems, and ease of implementation and maintenance. Its drawbacks are limited scale-up potential and a central failure point. The authors used SEMINT to complete a number of analyses that require special intervisibility computations of low-observable vehicles and high-resolution, ground-air combat involving helicopters. This report discusses the approach, the models integrated to date, and the lessons learned during implementation.