When we published our first edition we were apprehensive that our offspring might not survive beyond a first printing. Now, over ten years later, we are happy to have the opportunity to present our fourth edition. We are happy, also, to be able to report that our initial venture has, in the best tradition of American enterprise, shown an impressive growth rate, even though its contributions to aggregate employment and to the gross national product remain less than overwhelming. Humor aside, the important thing is that the book has proved helpful and instructive to numerous students and teachers, and it has, we believe, helped to support the mission of better economic understanding. That is what justifies its existence. And that is what has justified the considerable effort that went into the preparation of this fourth edition.