The sharing economy appears to have been growing rapidly. This paper contributes to the debate about its definition and measurement through an analysis of interviews conducted with UK platforms identifying themselves as part of the sharing economy. We conclude there are common features that enable a sufficiently clear definitional boundary, namely peer-to-peer digital matching and greater utilisation of under-used assets or skills. We find that the larger sharing economy platforms reduce costs and entry barriers for smaller platforms, contributing to a rich ecosystem. This implies a useful definition should include business-to-business peer-matching transactions, as well as business-to-consumer transactions. In addition to their economic impacts – transactions that would not otherwise occur, lower consumer prices and additional choice, the scope to earn additional income in a flexible manner, and the greater use of assets with spare capacity – all the interviewees expressed overt non-financial motivations, such as positive environmental impact, contributing to the community, and building trust. We argue this common intrinsic motivation means measurement of the sharing economy for some purposes should also include those platforms which enable free rather than monetary exchanges.