Pumpelly v. Green Bay Company was a landmark case in the United States Supreme Court. The case involved a dispute between a lumber company and a railroad company over the rights of the lumber company to use the railroad's tracks. The lumber company, Pumpelly, argued that the railroad company, Green Bay, had violated the terms of their contract by refusing to allow them to use the tracks. The Supreme Court ruled in favor of Pumpelly, finding that the railroad company had violated the contract and that Pumpelly was entitled to damages. The Court's decision in Pumpelly v. Green Bay Company established the principle of "equitable estoppel," which holds that a party cannot deny the terms of a contract if they have already accepted the benefits of the contract. This ruling was significant because it established a precedent for the enforcement of contracts in the United States. The Court's decision also established the principle of "promissory estoppel," which holds that a party cannot deny the terms of a contract if they have already made a promise to fulfill the terms of the contract. The Court's decision in Pumpelly v. Green Bay Company was a landmark ruling that established important principles of contract law in the United States. The ruling established the principle of equitable estoppel, which holds that a party cannot deny the terms of a contract if they have already accepted the benefits of the contract. The ruling also established the principle of promissory estoppel, which holds that a party cannot deny the terms of a contract if they have already made a promise to fulfill the terms of the contract. These principles are still used today in contract law cases.

In Pumpelly v. Green Bay Company, the Supreme Court was asked to determine whether a federal court had jurisdiction over an action brought by a citizen of New York against a Wisconsin corporation for breach of contract. The majority opinion held that the case did not fall within any of the enumerated categories in which Congress had granted federal courts jurisdiction and thus concluded that it lacked authority to hear such cases. Justice Field dissented from this decision, arguing that although Congress may have intended only certain types of cases to be heard in federal courts, its language was broad enough to encompass all civil actions between citizens of different states regardless of their nature or subject matter. He further argued that if Congress wanted to limit its grant of power then it should have done so explicitly rather than leaving it open for interpretation by the judiciary.