||Reductionism is a family of ontological, epistemological, methodological, linguistic, and explanatory views that apply to areas of science, mathematics, logic, and philosophy. Here the focus is upon reductionism in the sciences. The term “reductionism” suggests different concepts to different individuals, yet there are some common themes. Thus, reductionism in ontology typically involves a simplification in terms of what is more fundamental, ultimately the things in basic physics. Moreover, this simplification is typically achieved by means of unifying relations such as identity or composition or constitution, which contrast with the non-unifying attitudes of replacement or elimination if one thinks that the ontology of a special science does not reduce. Furthermore, the unifying relations might hold between partial or full domains of objects designated by the pertinent sciences, say, a partial reduction of special science properties but not the properties of phenomenal consciousness, or a partial reduction of particulars (for token reduction) but not all their properties (for type reduction). As well, the subsequent versions of partial and full reductionism may apply to various fields of science, such as the social sciences, or cognitive science, or biology. Finally, these many versions of reductionism contrast with nonreductive views regarding the objects or entities described in the sciences, such as ideas about emergence, supervenience, and realization.