The fundamental idea of Design by Contract is worked out for the specification and design of concurrent distributed interactive systems including properties of real-time behavior. We use interface predicates for specifying systems and forming system architectures by composition. We show how to begin with an interface predicate specifying a nucleus of behavioral properties and how to extend and refine it schematically by properties that hold for all implementations such as strong causality and full realizability. The goal is a modular specification and design approach for concurrent distributed interactive systems following the principles of modularity, encapsulation and information hiding.