The Supreme Court has increasingly considered a particular kind of argument: that it should avoid reaching decisions that would “open the floodgates of litigation.” Despite its frequent invocation, there has been little scholarly exploration of what a floodgates argument truly means, and even less discussion of its normative basis. This Article addresses both subjects, demonstrating for the first time the scope and surprising variation of floodgates arguments, as well as uncovering their sometimes-shaky foundations. Relying on in-depth case studies from a wide array of issue areas, the Article shows that floodgates arguments primarily have been used to protect three institutions: coordinate branches of government, the state courts, and the federal courts themselves. In the former two instances, the Court’s desire to avoid floods is supported by independent constitutional principles and doctrine, including separation of powers and federalism, lending these kinds of arguments a prima facie legitimacy. With regard to the final instance, however, the Court has relied on floodgates arguments solely to protect itself and the rest of the federal judiciary from what it sees as an excessive workload, raising difficult questions about separation of powers and the measures courts can take to ensure their ability to administer justice. The Article concludes by arguing for a presumption against court-centered floodgates arguments—positing that the Court should let the lower courts rely on alternative mechanisms, such as procedural rules and case-management techniques, to handle new claims instead of closing the courthouse doors to stave them off altogether.