The division of labour, whereby individuals within a group specialize in certain tasks, has long been appreciated as central to the evolution of complex biological societies. In recent years, several examples of division of labour in microorganisms have arisen, which suggests that this strategy may also be important in microbial species. In this Opinion article, we explore the set of conditions that define division of labour and propose that cooperation between different phenotypes is a defining feature of division of labour. Furthermore, we discuss how clarifying what constitutes division of labour highlights key evolutionary questions, including what form division of labour takes and why it is favoured by natural selection.