With the emergence of Web service technologies, it has become possible to use high level megaprogramming models and visual tools to easily build distributed systems using Web services as reusable components. However, when attempting to apply the Web service composition paradigm in practical settings, some limitations become apparent. First of all, all kinds of existing “legacy” components must be wrapped as Web services, incurring in additional development, maintenance, and unnecessary runtime overheads. Second, current implementations of Web service protocols guarantee interoperability at high runtime costs, which justifies the composition of only coarse-grained Web services. To address these limitations and support the composition of also fine-grained services, in this paper we generalize the notion of service by introducing an open service meta-model. This offers freedom of choice between different types of services, which also include, but are not limited to, Web services. As a consequence, we argue that service composition – defined at the level of service interfaces – should be orthogonal from the mechanisms and the protocols which are used to access the actual service implementations.