Erie doctrine

The Erie doctrine is a fundamental principle of civil procedure in the United States that requires federal courts to apply state substantive law in disputes that do not directly involve a federal question. This most commonly occurs when a federal court is exercising diversity jurisdiction, where the United States federal courts hear a case that is not a federal question. It also applies when supplemental jurisdiction is used for claims related to a federal question, or during an adversary proceeding in bankruptcy. Supplemental jurisdiction refers to federal courts hearing additional claims that it otherwise would not have jurisdiction over.

The doctrine follows from the Supreme Court landmark decision in Erie Railroad Co. v. Tompkins (1938). The case overturned Swift v. Tyson, which allowed federal judges sitting in a state to ignore the common law local decisions of state courts in the same state in diversity actions. It permitted federal courts to enforce a federal law instead of state law when the issue had not been previously addressed.