This Article addresses a topic of contemporary public policy significance: the optimal allocation of law enforcement authority in our federalist system. Proponents of “competitive federalism” have long argued that assigning concurrent enforcement authority to states and the federal government can lead to redundant expense, policy distortion, and a loss of democratic accountability. A growing literature responds to these claims, trumpeting the benefits of concurrent state-federal enforcement—most notably the potential for state regulators to remedy under-enforcement by captured federal agencies. Both bodies of scholarship are right, but also incomplete. What is missing from this rather polarized debate is a deep appreciation for how context matters. This Article moves beyond the abstract case for or against concurrent state-federal enforcement, and provides a systematic account of the variables that will influence its desirability in disparate regulatory settings. To illustrate the significance of these variables, the Article also provides an empirically-grounded case study of one of the most contentious areas of concurrent state-federal authority—securities fraud enforcement against nationally traded firms.