The vision of the Semantic Web is to reduce manual discovery and usage of Web resources (documents and services) and to allow software agents to automatically identify these Web resources, integrate them and execute them for achieving the intended goals of the user. Such a composed Web service may be represented as a workflow, called service flow. Current Web service standards are not sufficient for automatic composition. This paper presents different types of compositional knowledge required for Web service discovery and composition. As a proof of concept, we have implemented our framework in a cardiovascular domain which requires advanced service discovery and composition across heterogeneous platforms of multiple organizations. Categories and Subject Descriptors: I.2.4 [Knowledge Representation Formalisms and Methods]: Representation Languages General Terms: Management, Design