Questioning the widespread tendency to view academic spin-outs as a homogenous category, the paper explores typologies of these companies using a Penrosean conceptualisation of entrepreneurial activity. We initially identify five main types of business activities pursued by academic entrepreneurs, which we revise after analysing a database of Cambridge University spin-outs and real-time exemplars of emerging ventures. The refined typology takes into account the dynamic of the entrepreneurial process. As the business models of ventures evolve they may enter a different category of business activity. We conclude by discussing the academic and practical needs for a better understanding of the heterogeneity of spin-outs, the diversity of which has theoretical and policy implications.