The idea of service orientation envisions dynamic detection and execution of suitable Web services for solving a particular request. Most realization approaches pay only little attention to the client side of such architectures. We therefore promote a goal-driven approach: a client merely specifies the objective to be achieved in terms of a goal, and the system resolves this by automated detection, composition, and execution of Web services. Extending the WSMO framework, we present a model for describing goals as formalized client objectives that carry all information relevant for automated detection and execution of Web services. This paper explains the design of the goal model, specifies the formal descriptions of goals, and demonstrates the model within an illustrative example.