This Article posits that two significant problems in the Supreme Court’s personal jurisdiction case law have led to incoherent and irreconcilable results in cases involving individual and corporate defendants. First, the Court has imposed substantive due process limitations on a state’s assertion of personal jurisdiction without ever explaining why such limitations are constitutionally required. Second, in the absence of clearly enunciated principles of substantive due process, the Supreme Court has relied on poorly defined categories of the types of contacts that would satisfy the substantive due process requirement. In Part I, the Article discusses the history of the U.S. Supreme Court’s substantive due process limitations on personal jurisdiction and, in particular, the standards for corporate-activities-based jurisdiction before the Court’s recent cases on that issue. Part II discusses the Court’s failure to provide a convincing theoretical justification for imposing substantive due process limitations on personal jurisdiction. It also discusses the consequences of that failure in three doctrinal areas of personal jurisdiction law, the traditional basis of service on an individual in the forum state, specific jurisdiction and corporate-activities-based jurisdiction. Part III then analyzes in detail the four recent Supreme Court cases on personal jurisdiction, and discusses the mistaken assumptions underlying those decisions. Part IV explains how the Court’s personal jurisdiction rules, as a whole, suffer from theoretical bareness, the ambiguity of the substantive due process categories of jurisdiction, and the rigidity of the Court’s substantive due process analysis. Finally, in Part V, the Article offers some ideas for how the Court could begin to remedy the many problems with personal jurisdiction law.