To stay ahead in today’s competitive landscape service firms must continually innovate their offerings and processes (Thakur and Hale 2013). Spurred by this focus on innovation, there has been a tremendous growth in service firms over the past decade. Service innovation has been recognized as the engine of renewal and reason for the growth of the services sector. Although the concept service innovation is gaining increased attention, the question of what a service innovation really is remains unanswered. Traditionally, research on service innovation has separated radical and incremental innovations (see e.g. Gallouj and Weinstein, 1997), and product and process innovations (Vaux Halliday and Trott, 2010). This suggests that there are different conceptualizations that can be used to categorize service innovation. The basis of this research is a literature review. This systematic literature review is based on 879 empirical and conceptual articles used in order to identify conceptualizations of categories of service innovation. In total, 43 categories of service innovation were identified. These conceptualizations were analyzed to show what the benefits and drawbacks are with the different categories. We identify five unique conceptualizations of categories that emphasize different traits of service innovation (1) degree of change in the offering, (2) type of change in the content of the offering, (3) degree of newness in the offering, (4) type of change in the organization and (5) alternative means to an innovative offering. Our review of categories of service innovation provides evidence that the research field is diverse and moving in a number of alternative directions The results indicate that most conceptualizations of service innovation are in-ward focus and views innovations as something that is (only) new to the firm. In addition, the view that service innovations must have an effect is neglected, both in terms of effect on customer behavior and financial performance. Furthermore, this review shows that a service innovation can be anything from an improvement of a single service characteristic to the introduction of an offering that is new to the world. By showing the plethora of conceptualizations of service innovation it is possible to create a common platform to discuss and develop service innovations beneficial for the firm.