This book’s theme is “The Science of Service Systems,” yet there is substantial question about whether the definition and nature of service systems have been articulated adequately. This paper examines definitions of service and service system that could frame or otherwise influence future developments in service science and could have implications for what should and should not be included within service science. It argues that the initial development of service science should use straightforward definitions that are understandable, useful, broadly applicable, and teachable. It proposes a definition of service system that is different from the definition proposed in this book’s Call for Chapters and in the 2008 White Paper produced by a service innovation symposium attended by many leaders in the effort to create service science. In comparison with that definition, this paper’s alternative definition is more understandable, useful, broadly applicable, and teachable.