Courts and commentators have assumed that the Erie doctrine, while originating in diversity cases, applies in all cases whatever the basis for federal jurisdiction. Thus, when a federal court asserts jurisdiction over pendent state law claims through the exercise of supplemental jurisdiction in a federal question case, courts regularly apply the Erie doctrine to resolve conflict between federal and state law. This Article shows why this common wisdom is wrong. To understand why, it is necessary to return to Erie’s goals, elaborated over time by the U.S. Supreme Court. Erie and its progeny are steeped in diversity-driven policy concerns: concerns about unequal treatment based on state citizenship and forum shopping figure most prominently. T...