Pennoyer v. Neff was a United States Supreme Court case that addressed the issue of personal jurisdiction. The case involved a dispute between two Oregon residents, Jacob Neff and Nathan Pennoyer, over a piece of real estate. Neff had purchased the property from Pennoyer, but Pennoyer later attempted to reclaim it. Neff then sued Pennoyer in Oregon state court. Pennoyer, however, was not present in Oregon at the time of the suit and had not been served with process. The Supreme Court held that the Oregon court lacked personal jurisdiction over Pennoyer. The Court reasoned that due process requires that a defendant must have sufficient notice of the suit and an opportunity to be heard. Since Pennoyer had not been served with process, he had not been given the opportunity to be heard and the Oregon court lacked jurisdiction. The Court also held that a state court cannot acquire jurisdiction over a nonresident defendant simply by publishing notice of the suit in a newspaper. The decision in Pennoyer v. Neff established the principle that a court must have personal jurisdiction over a defendant in order to render a valid judgment. This principle has been applied in numerous cases since then and remains an important part of the American legal system.

In the case of Pennoyer v. Neff, the Supreme Court was tasked with determining whether a judgment rendered by an Oregon court against a non-resident defendant could be enforced in another state. The majority opinion held that it could not, as due process had not been served on the defendant and he had no opportunity to appear before or defend himself in front of the Oregon court. Justice Field dissented from this decision, arguing that service upon an agent appointed by law for such purpose constituted sufficient notice under due process requirements. He argued further that even if service was insufficiently made, there were other remedies available which would have allowed for proper notification and thus enabled enforcement of judgments across state lines without violating constitutional rights to due process.