The central question in this line of investigation is whether – and if so, how – philosophical theories of natural kinds can contribute to our understanding of classificatory practices in the sciences, in particular the biological sciences. Often, philosophical discussions of the notion of natural kinds are carried out in an a priori manner. That is, accounts of what it is to be a natural kind are developed on the basis of a priori considerations, often of a metaphysical nature, and concrete cases from the sciences subsequently are measured against the account that was developed independently. Because of its a priori nature, however, it is not clear to what extent results from such traditional philosophical work on natural kinds can be used to illuminate the successful use of classifications in the sciences (or elsewhere, for that matter). My research aims at developing a thoroughly naturalistic theory of natural kinds that can do justice to the various kinds and classifications that feature in the special sciences. The project starts from examinations of concrete instances of classification in biological science as well as results from recent work on the epistemology and metaphysics of natural kinds, and expands from there into other areas of science.