Today's distributed systems are characterized by interactions, often complex, between many different hardware/software components cooperating and exchanging information. To reduce the complexity of the development of interactive systems, experts employ descriptions, or specifications, of a given system's behavior and/or structure. This book provides a mathematical and logical foundation for the specification and development of interactive systems based on a model that describes systems in terms of their input/output behavior. Based on the model, the authors build a basic method, called FOCUS, that enables interactive systems to be described by characterizing their histories of message interaction. The book progresses from an introduction and guided tour of FOCUS through streams, specifications and their properties, and behavioral, interface, and conditional refinements.