As Web services composition arouses a growing interest, most research works address implementation and execution issues. Therefore, many composition languages (BPEL, XLANG, WSFL, WSCI, to name a few of them) have been proposed in the past few years. However, a weakness of these languages is that they are difficult to use in early stages of development, such as specification. In this paper, an extension to UML 2.0 called ”UML-S: UML for Services ” is introduced. UML-S allows for a Model Driven Engineering (MDE) of Web services and their interactions. 1