Publisher Summary A service of the system under development (SuD) is an external interaction with a triggering event that delivers an identifiable added value to the environment. A service description should clarify what the added value delivered to the environment is and which assumptions about the environment, if any, have been made. Typically, if these assumptions are not satisfied by the environment, then it may not be desirable for the SuD to provide the service. A service description should in addition state what the triggering event of the service is. This chapter describes the format in which services are described and gives some guidelines. Services are described by giving four pieces of information: name, triggering event, delivered service, and assumptions.