Longstreth v. Pennock was a United States Supreme Court case that dealt with the issue of whether a party could be held liable for a breach of contract when the contract was not in writing. The case involved two parties, Longstreth and Pennock, who had entered into an oral agreement for the sale of a piece of property. Longstreth had agreed to pay Pennock $2,000 for the property, and Pennock had agreed to deliver the deed to the property. The Supreme Court held that the oral agreement between the parties was enforceable, and that Longstreth could be held liable for breach of contract. The Court reasoned that the parties had entered into a binding agreement, and that the lack of a written contract did not invalidate the agreement. The Court also noted that the parties had acted in reliance on the agreement, and that Longstreth had paid the agreed-upon price for the property. The Court's decision in Longstreth v. Pennock established that oral contracts are enforceable, and that parties can be held liable for breach of contract even if the contract is not in writing. This decision has been cited in numerous subsequent cases, and has been used to support the enforceability of oral contracts.

In Longstreth v. Pennock, the United States Supreme Court was tasked with determining whether a contract between two parties that had been partially performed could be enforced by a court of equity. The majority opinion held that such contracts were not enforceable in equity because they lacked consideration and mutuality of obligation. However, Justice Field dissented from this ruling on the grounds that it failed to take into account the fact that one party had already performed their part of the agreement and should therefore be entitled to some form of relief for their efforts. He argued further that if courts refused to recognize these types of agreements then individuals would have no incentive or protection when entering into them since there would be no guarantee they will receive anything in return for fulfilling their obligations under such an arrangement. As such, he concluded that courts should provide equitable remedies where appropriate so as to ensure fairness and justice is served in all cases involving contractual disputes regardless of how much has been completed by either side prior to litigation being initiated.