The Intelligent Room (IRoom), an Intelligent Environment (IE) at the MIT Aritificial Intelligence (AI) Laboratory (Coen, 1998), is an experiment in human-computer interaction (HCI). Most of our research centers on a particular implementation of the IRoom, a conference room enhanced with cameras, microphones, and other sensors. Part of our HCI research is in enabling the Intelligent Room to “intelligently” react to human user behavior. This paper describes several design principles for an ?intelligent? system to help fill that role. Such a system is called a reactive behavioral system.