This book is about the economic theory of consumer behavior and its uses in economic analysis. It is about the tools and language of utility theory and their application to a field that ranges from empirical work on commodity demand to abstract questions of social choice. The basic theory is the familiar one, although we have made extensive use of cost functions and related “duality” concepts to present it in a way that simplifies what is often seen as difficult or inaccessible material. This, and the range of subject matter, broader than any previous book on consumer behavior, are the most distinctive features of the book. Our main purpose in writing it is to provide in. one place a complete toolbox of utility theory together with a demonstration of the power of these tools in action over a wide front of economics. Although the use of utility theory runs as a common thread throughout the book, only a fraction of the space deals with the standard textbook model of choice subject to a linear budget constraint. In recent years, important work has been done in many areas of economics by applying consumer theory to nonstandard situations, for example, to discrete choice, to rationing, to labor supply, to fertility, to quality choice, to choice with complex nonlinear budget constraints resulting from tax and benefit systems, liquidity constraints, uncertainty, and so on. Most of this work emphasizes careful modeling of the constraints that consumers face, including the statistical and econometric consequences.