304 U.S. 64 (1938).
A landmark United States Supreme Court decision wherein the Court held that federal courts in diversity actions were to apply the substantive law (both statutory and common law) of the forum state to the state causes of action. Prior to Erie, federal courts in diversity cases would look to state statutes, but not to state judicial decisions establishing the common law of the state. See Swift v. Tyson, 41 U.S. 1 (1842).
In Erie, the Court overruled Swift v. Tyson, and held that the Rules of Decision Act (now codified at 28 U.S.C. § 1652) requires federal courts in diversity cases to apply both state statutes and decisional law. But the Erie Court did not require federal courts to pretend to be state courts or to apply state announced rules to all questions. Rather, federal courts in diversity cases were required to apply state rules of law, whether statutory or judicially announced “[e]xcept in matters governed by the Federal Constitution or by acts of Congress.” Id. (emphasis added).
In the years since Erie, federal courts have had to determine whether certain matters are governed by acts of Congress — e.g., Federal Rules of Civil Procedure — or by substantive protections of state law — e.g., state anti-SLAPP statutes, with their procedures for early dismissal and limitations on discovery.