The Erie doctrine is a fundamental legal doctrine of civil procedure in the United States which mandates that a federal court called upon to resolve a dispute not directly implicating a federal question (most commonly when sitting in diversity jurisdiction, but also when applying supplemental jurisdiction to claims factually related to a federal question or in an adversary proceeding in bankruptcy) must apply state substantive law.
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.
Scope
There are two main objectives of the Erie decision: (1) to discourage forum shopping among litigants, and (2) to avoid inequitable administration of the laws. Broadly speaking, the second objective is sometimes referred to as "vertical uniformity" and is rooted in the idea that in a given state, the outcome of the litigation should not be grossly different just because a litigant filed a claim in a state court rather than a federal court or vice versa.
The Erie doctrine today applies regardless of how the federal court may hear a state claim. Whether the federal court encounters a state law issue in diversity jurisdiction, supplemental jurisdiction, or bankruptcy jurisdiction, the federal court must honor state common law when deciding state law issues.
In effect, when the U.S. Constitution does not control and Congress has not legislated (or cannot legislate) on a topic, then the laws of the states necessarily govern and state judge-made rules are equally binding on the federal courts as state statutes.
The federal court must determine if either 1) state law is clear as to the case in controversy, or 2) if not, then has the state's highest court ruled specifically on a similar case. If so, the state law or court ruling must be followed. But if not, then the federal court must determine how the state's highest court would potentially rule on a matter: for example, it may look to state appellate courts to see how they ruled, and if the state chose not to hear further appeals, the federal court could determine that the high court agreed with the appellate courts. The determination is called an "'Erie guess", though the term "guess" is a misnomer as the federal court must make a reasoned determination in its ruling.
Alternatively though, the court may elect to certify the question to the state's highest court for a ruling on state law in the matter. Some states, however, do not allow Federal District Courts to certify questions, only the Supreme Court or Federal Circuit Courts of Appeals.
Origin
The Erie case involved a fundamental question of federalism and the jurisdiction of federal courts in the United States. In 1789, the Congress passed a law still in effect today called the Rules of Decision Act (), which states that the laws of a state furnish the rules of decision for a federal court sitting in that state. Thus, a federal court in Texas, hearing a case based on diversity (as opposed to a federal question), has to follow the laws of the applicable state in resolving a case before it.
Swift v. Tyson
The Supreme Court's decision in Swift v. Tyson had defined the laws of the state as meaning only laws passed by legislatures of that state (though Justice Joseph Story writing for the court suggested that federal courts should pay special attention to how the "local tribunals" of a state would resolve a dispute). Thus, on issues of "general common law", a federal court was free to ignore decisions by a state's highest court.