Taxonomy is the theory and practice of biological classification.

Biologists have so far described approximately one million species of living plants and animals, and perhaps a further quarter million extinct fossil species. Estimates vary for the number of species that exist but have not yet been described: there may be between 10 and 35 million of them. Describing a species is a formalized activity, in which the taxonomist has to compare specimens from the new species and other, similar species, and then explain how the new species can be distinguished; the description also has to be published.

Describing species is the most important task of taxonomists, but it has no particular connexion with evolutionary biology. The evolutionary interest of classification begins at the next stage with taxonomic hierarchy.

The image opposite is of a number of beetle specimens from a museum collection. Of the million or so species classified, approximately a third are types of beetle.