This book argues that the core idea of intertheoretic reduction is still defendable. The book is divided into three parts. In Part One, comprising five chapters, a positive account of reduction is presented. First, Nagel''s classic account of reduction is considered. Next, the problems of meaning variance and inconsistency are critically discussed. Finally, the notion of "multiple realization" and its bearing on intertheoretic reduction will be analyzed. Part Two, comprising three chapters, focuses on inadequate accounts of reduction: explanatory reduction, those accounts which define reduction in terms of supervenience and functional reduction. Finally, in Part Three, I return to the version of reduction that I defend. This has two importansat components: approximation and non-formal conditions. First, I sketch a portrait of this account, and then I consider some of its features, such as its aim and relata, its non-formal nature and its direction.