 The nature of species is controversial in biology and philosophy. Biologists disagree on the definition of the term ‘species,’ and philosophers disagree over the ontological status of species. Yet a proper understanding of species is important for a variety of reasons. Species are the fundamental taxonomic units of biological classification. Environmental laws are framed in terms of species. Even our conception of human nature is affected by our understanding of species. This entry discusses a number of philosophical issues concerning biological species.