Antimonopoly, meaning opposition to the exclusive or near-exclusive control of an industry or business by one or a very few businesses, played a relatively muted role in the history of the post-1945 era, certainly compared to some earlier periods in American history. However, the subject of antimonopoly is important because it sheds light on changing attitudes toward concentrated power, corporations, and the federal government in the United States after World War II. Paradoxically, as antimonopoly declined as a grass-roots force in American politics, the technical, expert-driven field of antitrust enjoyed a golden age. From the 1940s to the 1960s, antitrust operated on principles that were broadly in line with those that inspired its creation in the late 19th and early 20th century, acknowledging the special contribution small-business owners made to US democratic culture. In these years, antimonopoly remained sufficiently potent as a political force to sustain the careers of national-level politicians such as congressmen Wright Patman and Estes Kefauver and to inform the opinions of Supreme Court justices such as Hugo Black and William O. Douglas. Antimonopoly and consumer politics overlapped in this period. From the mid-1960s onward, Ralph Nader repeatedly tapped antimonopoly ideas in his writings and consumer activism, skillfully exploiting popular anxieties about concentrated economic power. At the same time, as part of the United States’ rise to global hegemony, officials in the federal government’s Antitrust Division exported antitrust overseas, building it into the political, economic, and legal architecture of the postwar world. Beginning in the 1940s, conservative lawyers and economists launched a counterattack against the conception of antitrust elaborated in the progressive era. By making consumer welfare—understood in terms of low prices and market efficiency—the determining factor in antitrust cases, they made a major intellectual and political contribution to the rightward thrust of US politics in the 1970s and 1980s. Robert Bork’s The Antitrust Paradox, published in 1978, popularized and signaled the ascendency of this new approach. In the 1980s and 1990s antimonopoly drifted to the margin of political debate. Fear of big government now loomed larger in US politics than the specter of monopoly or of corporate domination. In the late 20th century, Americans, more often than not, directed their antipathy toward concentrated power in its public, rather than its private, forms. This fundamental shift in the political landscape accounts in large part for the overall decline of antimonopoly—a venerable American political tradition—in the period 1945 to 2000.