The automation of cross-organizational business processes is one of the most important trends of the information age. Instead of a tight integration however, collaborating organizations rather strive for a loose coupling of their information systems. Supporting this objective, the Architecture of Interoperable Information Systems (AIOS) represents a means for the comprehensive description of loosely coupled, interoperating information systems and for the systematic, model-based enactment of collaborative business processes. To this aim, it combines concepts from the areas of enterprise modeling, collaborative business and Service-oriented Computing. At the core of the architecture lies the Business Interoperability Interface, which describes the information system boundaries of one organization to its collaboration partners and connects internal and external information systems. Detailed procedure models specify the usage of the AIOS; its application to an example scenario as well as prototypes that implement core aspects of the AIOS exemplify the method. This book addresses researchers as well as practitioners interested in the areas of organizational interoperability and the modeling and enactment of collaborative business processes.