A rule of contractual construction that prohibits parties in litigation from introducing extrinsic evidence of prior or contemporaneous agreements, negotiations, or representations to modify, supplement, or contradict a written contract. The rule only applies to written contracts that contain the complete and final agreement between the parties to the contract. These contracts are referred to as integrated contracts and typically include integration clauses (also referred to as merger clauses).

Even where there is an integrated contract, the rule is subject to certain exceptions. For example, a party can use extrinsic evidence to: