In this paper, we address some of the challenges raised by the emerging service-oriented computing paradigm in what concerns the ability to define dynamic interactions between core services for flexible and agile business processes. We claim that, from this point of view, service interaction and composition is well beyond the reach of object-oriented and component_ based techniques. We argue instead for the use of architectural modelling techniques that promote the externalization of coordination mechanisms. We show how what we call composition laws and interfaces can be used to define the coordination logic according to which the behavior of a business process can be described in terms of interactions with given partners. These primitives provide a business modelling level that can be mapped onto the specifications that are being proposed for web services, e.g., BPEL, WS-Coordination or WS-Transaction.