U.S. Supreme Court delivers its opinion in Erie Railroad Co. v. Tompkins and overturns a century of federal common law.

The Supreme Court of the United States (SCOTUS) is the highest court in the federal judiciary of the United States. It has ultimate appellate jurisdiction over all U.S. federal court cases, and over state court cases that turn on questions of U.S. constitutional or federal law. It also has original jurisdiction over a narrow range of cases, specifically "all Cases affecting Ambassadors, other public Ministers and Consuls, and those in which a State shall be Party." In 1803, the Court asserted itself the power of judicial review, the ability to invalidate a statute for violating a provision of the Constitution via the landmark case Marbury v. Madison. It is also able to strike down presidential directives for violating either the Constitution or statutory law.

Erie Railroad Co. v. Tompkins, 304 U.S. 64 (1938), was a landmark U.S. Supreme Court decision in which the Court held that the United States does not have a general federal common law and that U.S. federal courts must apply state law, not federal law, to lawsuits between parties from different states that do not involve federal questions. In reaching this holding, the Court overturned almost a century of federal civil procedure case law, and established the foundation of the modern law of diversity jurisdiction.