Hermaphroditism is common, occurring in >90% of plant genera (Renner and Ricklefs, 1995), >70% of animal phyla (Jarne and Auld, 2006), and being present also in many other multicellular taxa, such as volvocine algae (Coleman, 2012) and arguably also in the fungi (Nieuwenhuis and Aanen, 2012) (see ‘Glossary’ for related terms used in diverse organismal groups). The key distinguishing feature of hermaphroditism is that each individual can (at least potentially) gain fitness through both male and female reproduction, either by adopting the two sexes sequentially or simultaneously. We first outline the evolutionary context for thinking about hermaphroditism, focusing on (1) it being a frequent outcome following the evolution of ‘anisogamy,’ (2) what might be the evolutionary advantages of maintaining two individual routes to fitness, and (3) how local competition drives individual investment decisions between the two sex functions (i.e., ‘sex allocation’). We then apply this framework to highlight some of the key differences in the evolutionary biology of sex in hermaphrodites compared to gonochorists, firstly for sequential hermaphrodites and then for simultaneous hermaphrodites. Finally, we provide an outlook, and argue that studying sex in hermaphrodites is important for gaining a comprehensive general picture of the consequences of the evolution of anisogamy.