 The philosophical discussion of reduction-relations in the sciences was inspired by cases such as the alleged reduction of Newtonian mechanics to relativity theory, of chemistry to atomic physics, and of gas laws to statistical mechanics. However, in what respects these actual episodes of theory change count as reductions is a complex matter. Many factors might be relevant, not merely logical and metaphysical relations but also epistemic, psychological, and pragmatic ones. Questions also arose about the generalizability of these cases and their role in scientific progress or change, which led to the idea that an appropriate model of reduction should mainly focus on cases of actual theory succession. The main target in the reduction debate within the philosophy of science, however, was not a reconstruction of the process of scientific change, but rather an explication of the relation between successive theories that justified the relevant change, i.e., relations alleged to hold between pairs of theories such as Newtonian mechanics and relativity theory, or thermodynamics and statistical mechanics. In this context, the relata of reductions were conceived of as theories or models, rather than properties, events, or kinds. Thus, they are often referred to as ‘theory reductions’.