The innovative reapplication of a multi-agent system for human-in-the-loop (HIL) simulation was a consequence of appropriate agent oriented design. The use of intelligent agents for simulating human decision making offers the potential for analysis and design methodologies that do not distinguish between agent and human until implementation. With this as a driver in the design process the construction of systems in which humans and agents can be interchanged is simplified. Two systems have been constructed and deployed to provide defence analysts with the tools required to advise and assist the Australian Defence Force in the conduct of maritime surveillance and patrol. The systems both simulate maritime air operations. One utilises intelligent agents to provide models of tactical decision making, the other provides the same environment but provides a set of user interfaces to allow air force flight crews to participate in human in the loop simulation by replacing the intelligent agents. The experiences gained from this process indicate that it is simpler, both in design and implementation, to add humans to a system designed for intelligent agents than it is to add intelligent agents to a system designed for humans.