 The philosophical study of reduction links up with several key issues in the philosophy of science. As reduction involves relating (broadly speaking) one “body of scientific knowledge” to another, it presupposes an account of the relevant epistemic units to be related (e.g.,  theories,  laws,  models,  mechanisms,  concepts,  or disciplines)—how should these be understood? Since the reduction of one theory to another usually presumes that somehow the reducing theory explains the principles of the reduced theory, reduction is also related to the issue of  scientific explanation.  For example, if a more fundamental theory fully reduces another theory or even reduces several theories,  unification  (of some sort) results. The situation where a historically earlier theory is reduced to a later theory is an intuitive case in which  scientific progress  might occur. Arguments against the existence of this type of historical reduction have maintained that terms occurring in both the precursor and descendant theory have different meanings  (“incommensurability”).  This indicates that questions about reduction also connect to the meaning of scientific terms and conceptual change. Therefore, accounts of reduction often involve commitments made about other, coordinating notions and their adequacy depends in many cases on these related philosophical commitments. This has led some philosophers to pursue an account of reduction in biology that is as uncommitted as possible regarding the precise construal of general notions such as “law” and “explanation” (Sarkar 1998; cf. Brigandt 2013a), or to specifically highlight how a particular package of general notions travels together and skews our perspective on the shape of epistemic reduction in scientific practice (Godfrey-Smith 2008).