This Article unpacks the state court supervisory power by mapping its sources, applications, and limits. The supervisory power has a basis in all fifty state constitutions and enables supreme courts to oversee their judiciary’s workload and operations. But as this Article shows, high courts are using this power beyond the humdrum of judicial administration to enhance substantive rights and remedies, facilitate their law development and agenda-setting capabilities, and mediate interbranch frictions. This Article’s core claim is that these more expansive applications of the supervisory power are generally defensible based on the evolution of state judiciaries and supreme courts’ unique roles in state governments.