The ability to develop flexible business applications is one of the ultimate objectives behind the use of Web services. Before taking part in such applications, each Web service should be modeled so that service engineers can monitor its execution and identify and address design problems early on. The authors propose a novel approach for modeling Web services that distinguishes operational behavior, which defines the business logic underpinning the Web service's functioning, and control behavior, which guides the operational behavior's execution progress by identifying the actions to take and the constraints to satisfy. The authors' prototype system assists service engineers in specifying, enforcing, and monitoring these behaviors, thereby achieving a better design. © 2006 IEEE.