Federal courts in diversity jurisdiction cases must apply the law of the states in which they sit, including the judicial doctrine of the state's highest court, where it does not conflict with federal law. There is no general federal common law. Decided April 25, 1938 (argued January 31, 1938). Opinion of the Court: Louis Brandeis → go there Dissenting: Butler. Topic: Other. Erie Railroad Co. v. Tompkins, 304 U.S. 64 (1938), is a landmark decision of the Supreme Court of the United States which 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 any lawsuit between parties from different states that does not involve federal questions. The Court's decision overturned nearly a century of case law on federal civil procedure and established the foundation of the modern law of diversity jurisdiction. Wiki link