Towards a Theory of Services Wolfgang Reisig Humboldt-Universit¨ at zu Berlin, reisig@informatik.hu-berlin.de Abstract. Service-oriented Computing and Service-oriented Architec- tures aspire to better exploit existing middleware technologies. To this end, a more flexible, platform independent software design methodol- ogy is suggested, to develop rapid, low cost, interoperable, evolving and massively distributed software systems. Intended application areas in- clude 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 1 Introduction “Constantly growing complexity of information systems in industry and society already today approach the limits of our planning capacity. A formally founded tool box for planning, design development and operation of such systems is ur- gently needed. Methods are needed to determine the requirements at information systems. Likewise, we need formal concepts to describe, specify and analyze such systems, and to assess their behavior.” This quote dates back almost 30 years, published as the first section of the introduction to the volume “Formal Models for Information systems” [1], authored by Heinrich Christian Mayr. Many of his demands still remain to be fulfilled adequately. A promising candidate to ad- equately address those problems is the emerging paradigm of Service Oriented Computing (SOC). The basic notion of this paradigm are services. A service is an autonomous, platform independent component; frequently a software com- ponent. The most fundamental aspect of a service is its ability to communicate with other services. The functionality of a service can broadly vary, from quite simple activities such as answering a question, to involved exchanges of messages, as they occur in business processes, involving many partners and resources and including many alternatives, variants and iterations. A typical example of a service was a travel agent, communicating with other services such as flight carriers , hotels, ticket offices, etc. A client of a travel agent may want, say, to book a weekend flight from Klagenfurt to Berlin, to attend a Philharmonics performance. He would likewise behave as a service.