Following the introduction in Part I, Part II briefly examines structuralism as a theory underlying antitrust enforcement. The premise of structuralism was not high transaction costs, but rather the high costs of resource movement generally. Part III turns to barriers to entry or rival expansion, looking particularly at the differing definitions provided by Harvard and Chicago School economists and showing why the Harvard definition is superior for antitrust purposes. Part IV contains a brief discussion of antitrust's two principal tests for welfare, total welfare and consumer welfare, and how they relate to our assumptions about the costs of movement. Typically implicit in the argument for a general welfare test is that producer gains from a practice will move to consumers, provided that markets are competitive and other costs of movement do not serve to limit their flow. Implicit in defenses of a consumer welfare test is strong doubt that this will occur.