Abstract. Service-oriented Computing and Service-oriented Architectures aspire to better exploit existing middleware technologies. To this end, a more flexible, platform independent software design methodology is suggested, to develop rapid, low cost, interoperable, evolving and massively distributed software systems. Intended application areas include electronic commerce, information systems, supply chain control, and many other areas. Time is ripe to underpin this effort by theoretically well-founded concepts to model services, and to analyze such models. In this paper we suggest first proposals and principles for a comprehensive theory of services. Key words: Service-oriented Computing, Service Models, Theory of Services