Recent history of antitrust enforcement is often explained through the rise in the influence of economic efficiency as the goal of enforcement, which has led to an increased tolerance for all but the most direct horizontal anticompetitive practices, such as price-fixing and bid rigging. The history of antitrust enforcement is better understood, however, as the evolution of three enforcement conceptions. Until the mid-1960's, the emphasis was advocacy, which involved zealous prosecution to produce just outcomes. By the mid-1970's, enforcement practice tended to follow academic standards, which involved the selection of cases for prosecution that would achieve the best outcome according to pertinent intellectual standards. A winnable case that would produce harmful consequences according to these standards should not be brought. The 1980's saw a shift toward a democratic motivation guided by elected officials or appointees. Under this philosophy, the prosecutor should bring the kinds of cases the majority of the people prefer. In practice, this means prosecutors bring only those cases elected representatives want brought. Each of these conceptions holds philosophical legitimacy and generates rewards to proponents. Although the democratic motivation for bringing cases apparently renders prosecution vulnerable to political influences, arguments for a policy position on the basis of pure private gain are unlikely to be persuasive. Prosecution requires legitimacy through association with a theory of the general public good. Still, the formulation of policy positions expressed in terms of social benefits indicates a cost to policy advocates of ignoring considerations of ethics and justice. If so, there is reason to hope that the best outcome will eventually surface in the public debate. 17 notes and 27 references