Cook v. Tullis was a United States Supreme Court case that dealt with the issue of whether a state court had the authority to grant a new trial in a case that had already been decided by a federal court. The case arose when the plaintiff, Cook, sued the defendant, Tullis, in a state court for breach of contract. The state court found in favor of Cook and awarded him damages. Tullis then appealed the decision to a federal court, which reversed the state court's decision and dismissed the case. Cook then sought a new trial in the state court, which the court granted. Tullis then appealed the state court's decision to the Supreme Court. The Supreme Court held that the state court did not have the authority to grant a new trial in a case that had already been decided by a federal court. The Court reasoned that the state court was bound by the decision of the federal court and could not grant a new trial. The Court also noted that the state court's decision was in conflict with the federal court's decision and that the state court had no authority to override the federal court's decision. The Court thus held that the state court's decision was invalid and reversed it.

In the case of Cook v. Tullis, the Supreme Court was asked to determine whether a contract between two parties that had been partially performed could be enforced by one party against another. The majority opinion held that such contracts were enforceable and thus allowed for recovery of damages from the other party in this particular case. However, Justice Field dissented from this decision on several grounds. He argued that since only part of the contract had been performed, it should not be considered binding as there was no consideration given or received by either side for its performance; furthermore, he noted that allowing partial enforcement would lead to uncertainty and confusion in future cases regarding what constitutes an enforceable agreement. Additionally, he stated that even if some form of consideration did exist between both parties at one point in time prior to any breach occurring, it would have expired once said breach occurred due to lack of mutuality between them going forward with their contractual obligations.