Service metadata plays a central role in Service Oriented Architecture (SOA)s. Explicit service metadata makes possible the decoupling of applications and supports dynamic selection and binding. Moreover it extends the basic interoperability model beyond the implicit agreement to use a set of common protocols, making possible advanced collaboration models based on the dynamic discovery of partner capabilities. Web services provide the beginning of an XML based instantiation of the SOA model, with major middleware vendors providing implementations on top of their current platforms. In this paper, we discuss how the use of middleware metadata in SOA environments leads to optimized interaction and increase application decoupling.