One of the most pressing issues in antitrust law involves “product hopping.” A brand-name pharmaceutical company switches from one version of a drug (say, capsule) to another (say, tablet). The concern with this conduct is that some of these switches offer only a relatively minor medical benefit while at the same time significantly impairing generic competition. The antitrust analysis of product hopping is complex. In the US, it implicates the intersection of antitrust law, patent law, the Hatch-Waxman Act, and state drug product selection laws. In fact, the behaviour is even more complex because it involves uniquely complicated markets characterised by buyers (insurance companies, patients) who are different from the decision-makers (physicians). This chapter introduces the relevant US laws and regulatory frameworks before exploring the five important litigated cases.