When designing automated systems that make decisions about when to allow or deny interruptions, the methods of professional interruption mediators are an important source of information. Administrative assistants are, by the nature of their jobs, expert interruption mediators. They make decisions every day about whether to allow interruptions to the person they support. We have conducted a series of interviews with administrative assistants whose ability has been publicly recognized. Based on their responses, we present a production-rule model of the decision process they use when deciding whether to deliver interruptions to the person they support.