Web Services will transform the web from a collection of information into a distributed computational device. In order to employ their full potential, appropriate description means for web services need to be developed. For this purpose, we define a fully-fledged Web Service Modeling Framework (WSMF) that provides the appropriate conceptual model for developing and describing web services and their composition (complex web services). In a nutshell, its philosophy is based on the following principle: maximal de-coupling complemented by a scalable mediation service.