What is a service? Is there a single notion behind this term? And, if various visions exist, how are they related? In particular, how to reconcile the internal vision – services as business processes – with the external one of services as publicly exposed functionalities? This paper presents a systemic approach to the study of services based on ontological analysis. Services, whose eventive nature distinguishes them from goods, are modeled as complex concatenations of states and processes. Special attention is given in this analysis to the notion of service commitment, which the service process depends on, and to that of responsibility, which is at the basis of the notion of delegation.