Consumer behavior is frequently presented in terms of preferences, on the one hand, and possibilities on the other. The emphasis in the discussion is commonly placed on preferences, on the axioms of choice, on utility functions and their properties. The specification of which choices are actually available is given a secondary place and, frequently, only very simple possibilities are considered. In this book, we shall have a great deal to say about preferences, and discussion of them begins in Chapter 2. We begin, however, with the limits to choice rather than with the choices themselves. Unlike preferences, the opportunities for choice are often directly observable so that, to the extent that variations in behavior can be traced to variations in opportunities, we have a straightforward and objective explanation of observed phenomena. It is our view that much can be so explained and that the part played by preferences in determining behavior tends to be overestimated. Hence, this first chapter considers what can be said about behavior without detailed consideration, of how choices are made. A large part of this book, from Chapters 2 to 9, works with one very special assumption about the opportunity set, namely that choices are constrained by fixed, known prices in such a way that the total value of the objects chosen should not exceed some predetermined total. In this case, we say that the consumer faces a linear budget constraint.