 This entry discusses a number of philosophical issues concerning species. The first issue is their ontological status. Are species natural kinds, individuals, or sets? The second issue concerns species pluralism. Monists argue that biologists should attempt to find the correct definition of ‘species.’ Pluralists disagree. They argue that there is no single correct definition of ‘species’ but a plurality of equally correct definitions. The third issue concerns the reality of species. Does the term ‘species’ refer to a real category in nature? Or, as some philosophers and biologists argue, is the term ‘species’ a theoretically empty designation? The fourth and final issue concerns how social values affect which definitions of ‘species’ biologists use. 