Antitrust enforcement in the United States, starting with the Sherman Antitrust Act of 1890, has long focused on product-market competition and its antithesis, the monopoly provision of goods and services. In contrast, courts and regulators have largely neglected monopsony, where there exists one or a small handful of purchasers of goods and services, including labor services. Whether through noncompete clauses or simple market power, employers dominating local labor markets can depress wages and weaken worker protections. The author blames neglect of this issue on legal theorists’ nearly exclusive attention to consumer welfare and economists’ assumption that labor markets are generally competitive. He might have added a role for history and path dependence: antitrust efforts focus on monopoly today because they’ve focused on it for over a century. Posner concludes by showing that this imbalance can be righted within the existing legal framework if those responsible for antitrust policy wake up to the problem.