The promise of Web services is to enable the composition of new distributed applications/solutions: when no available service can satisfy a client request, (parts of) available services can be composed and orchestrated in order to satisfy such a request. Service composition involves two different issues: the synthesis, in order to synthesize, either manually or automatically, a specification of how coordinating the component services to fulfill the client request, and the orchestration, i.e., how executing the previous obtained specification by suitably supervising and monitoring both the control flow and the data flow among the involved services.