Federal courts typically assert jurisdiction over state and territorial law claims when the litigating parties satisfy the diversity requirement or as pendant claims attached to a federal claim. When the state or territorial law is clear, federal courts are well-equipped to apply it. When the law is not well-established, federal courts are typically forced to make their best "Erie guess." Increasingly, however, federal courts certify potentially dispositive, but unsettled, questions of state or territorial law to the highest court in the state or territory for a definitive interpretation.

The U.S. Supreme Court has thrown its support behind certification as a procedural tool. As the U.S. Court of Appeals for the Second Circuit observed," in the eyes of the Supreme Court, the device of certification provides all the benefits of Pullman abstention (deference in a federal system to state courts on questions of state law and statutory interpretations that avoid constitutional difficulties), while reducing greatly its drawbacks (delay and cost)." Tunick v. Safir (discussing the court's decision in Arizonans for Official English v. Arizona).

Of course, certification is not without its detractors. In the Second Circuit, for example, Judge Van Graafeiland once chastised Judge Calabresi's penchant for the practice, arguing that it "overlooked or disregarded the burdens on the State courts and the consequent delays that often result." Tunick v. Safir.