This paper reviews how the functional and structural scope of technological innovation systems (TIS) are understood in the literature. We find that it is often unclear if the system function involves innovation, production or both, and a lack of agreement as to whether structural elements are social or social and technical. Since these issues risk hindering cumulative knowledge development and conceptual advancements, we argue that a clear and shared underlying system model is needed. Taking steps in this direction, we propose that the function of a TIS is to develop and shape a specific technology; that this technology can be understood as a production-consumption system; and that the structural elements of a TIS are social, technical and possibly ecological. In addition, we offer guidance to boundary-setting in empirical case studies. We hope that the paper will inspire continued conceptual development in the TIS community and beyond.