The second and third editions of this book contained a variety of changes and additions that built on the success of the first edition. These included a number of new topics, such as the extend of a market, revealed preference, Engel curves, advertising, the theory of comparative advantage, strategic trade policy, and incentive design in vertically integrated firms. But even more importantly, with the assistance of a number of able reviewers, we heavily revised the first eight chapters of the text, and updated many of the examples. Our primary goal in revising the book has always been to make the text as clear, accessible, and interesting as possible. The fourth edition continues this tradition. We have added some new material, but we have also revised much of the exciting text with the goal of improved clarity and accessibility.