The Semantic Web services has become a challenging research topic in the last half of decade. Various frameworks offer means to semantically describe all the related aspects of Semantic Web services, but the solutions to the heterogeneity problems, inherent in a distributed environment such as the Web, are still to be properly integrated and referred to from the main phases of the Web services usage. Both data and process heterogeneity, as well as the multitude of functionalities required and offered by semantic Web services’ requesters and providers hamper the usability of Web services, making this technology difficult to use. This paper emphasizes the role of mediators in a Semantic Web services architecture, illustrating how the mediators can enable the Semantic Web services usages in operations like discovery, invocation and composition.