A conceptual model helps to know, understand, communicate and make simulations on the kind of objects that it represents. Although the main concepts of service science have been deeply discussed by many authors, their conceptual modelling is lacking. Therefore, in this paper, we progressively model the concept of service system, which is incontestably the fundamental element of service science. We define and depict the generic components of service systems as well as the relations between them. We also apply the recursion principle in order to model more complex service systems, i.e., service systems composed of other services systems. The key contribution of this work, i.e., the conceptual model of a generic service system, will help researchers and practitioners to argue on service systems as well as on their key components and relations.