Erie Railroad Co. v. Tompkins 304 U.S. 64, S. Ct. 817, 82 L. Ed. 1188 (1938) Tompkins was walking along the railroad tracks in Pennsylvania when he was hit by an open railcar door. Tompkins brought a lawsuit in Federal Court in New York against the Erie Railroad for personal injuries. Under Pennsylvania common law, where the accident occurred, the railroad owed no duty to Tompkins (he was trespassing). So according to Pennsylvania law, Tompkins would have to show that they deliberately ran him over, which is a high standard to meet, and Tompkins probably wouldn't win. There was no explicit Statute in Pennsylvania codifying the law. Instead of suing in a Pennsylvania State Court, Tompkins noted that Erie was incorporated in New York. As a Pennsylvania resident, Tompkins sued in Federal Court on diversity jurisdiction. The Federal Trial Court found for Tompkins and awarded damages. Erie appealed. Under the Federal common law, the railroad had a duty to take care, so Tompkins didn't have to show that Erie deliberately ran him over. Basically, in the Federal court, Pennsylvania common law did not apply. Erie unsuccessfully argued that under Judiciary Act § 34 (what is now 28 USC § 1652), the Federal Court was bound to use Pennsylvania common law as the basis for their decision. The Federal Appellate Court affirmed the Trial Court. Erie appealed. The US Supreme Court reversed the decision. The US Supreme Court found that Federal courts did not have the power to make up general Federal common law when hearing State law claims under diversity jurisdiction. This is now known as the Erie Doctrine. So basically, the Court found that when you are suing in a Federal court, you have to use the State's common law, so Tompkins would have to prove the same elements in Federal court that he would have to prove if he had brought the case in a Pennsylvania State court. With this decision the Court overturned almost a century of Federal civil procedure law, and established the current standard for diversity jurisdiction in Federal courts. The previous standard was that the laws of the several States included only State Statutes. That meant Federal judges were free to ignore a State's common law and substitute their own rules. (See Swift v. Tyson (41 U.S. 1 (1842))) With this case, the Court overturned Swift, calling it, "an unconstitutional assumption of power by the Courts of the United States." §1652 currently says, "The laws of the several States, except where the Constitution, treaties, or statutes of the United States otherwise require or provide, shall be regarded as rules of decision in trials at common law, in the courts of the United States, in cases where they apply." Note that the Erie Doctrine only applied to substantive laws, not procedural laws. Procedural laws are usually minor things like what size paper should be used for motions and when to take lunch. One of the most important procedural laws is the Statute of Limitations. But look at Guaranty Trust Co. v. York (326 U.S. 9 (1945)), where it was held that the Erie Doctrine can apply to some procedural laws too. One interesting piece of trivia about this case is that it's the only one where the US Supreme Court declared something unconstitutional, but never bothered to cite the constitution! It's unclear from the ruling what part of the Constitution it violates. It's assumed they found it was a violation of the 10th Amendment. There is one exemption to the Erie Doctrine. When the United States is a party to a case, they do not follow State law, they are bound by Federal law. When there is no Federal Statute, you can use Federal common law. Part of this decision was probably because West came out with their Keynote system. Prior to West, most people thought that common law was mostly the same from State to State, but after West, it was pretty obvious that States differed greatly in their interpretation of common law.