Service innovations challenge existing offerings and business models, shape existing markets, and create new ones. Over the last decade, service research has shown increasing interest in the concept of innovation and should by now have reached maturity and created a strong theoretical basis. However, there is no coherent theoretical framework that captures all the facets of service innovation, and to move service innovation research forward, we must revisit the key assumptions of what an innovation is. To enable this, the present article addresses three fundamental questions about service innovation: (1) What is it and what is it not? (2) What do we know and what do we not know? and (3) What do we need to know to advance service research? By doing so, this article offers an updated and comprehensive definition of service innovation and provides a research agenda to suggest a path forward.