Interstate compacts hold tremendous promise for resolving tough public problems at the subnational level. They have also been promoted as one way that states can protect their sovereignty vis-à-vis the national government. But the rate at which states participate in such compacts varies widely. Thus, we ask: What explains a state's propensity to join national interstate compacts? Using time-series cross-sectional event count models of state compact participation from 1960–2000, we find that states join compacts to enhance their policymaking capacity and to substitute for policy action by the national government. We also find that the physical connection among states influences their preference for certain types of compacts. Isolated states show a preference for compacts that simply harmonize policies, while more proximate states are more likely to join compacts that effectively open their borders to other states. Contrary to the expectations of some observers, interstate compacts do not appear to be used strategicallyby states as a means to forestall federal preemption.