 This encyclopedia entry started with the observation that at an intuitive level the nature of species seems fairly obvious. But a review of the biological and philosophical literature reveals that our understanding of species is far from settled. The debate over the nature of species involves a number of issues. One issue is their ontological status: are species natural kinds or individuals? A second issue concerns pluralism: should we adopt species monism or species pluralism? A third issue is whether the term ‘species’ refers to a real category in nature. Even Darwin, it seems, doubted that ‘species’ refers to a real category in nature. Finally we looked at how social values may affect which definitions of ‘species’ biologists use. Throughout this entry we have seen that many questions concerning biological species are in part philosophical. 