Pacta sunt servanda

Pacta sunt servanda ("agreements must be kept") is a brocard and a fundamental principle of law which holds that treaties or contracts are binding upon the parties that entered into the treaty or contract. It is customary international law. According to Hans Wehberg, a professor of international law, "few rules for the ordering of Society have such a deep moral and religious influence" as this principle.

In its most common sense, the principle refers to private contracts and prescribes that the provisions, i.e. clauses, of a contract are law between the parties to the contract, and therefore implies that neglect of their respective obligations is a violation of the contract. The first known expression of the brocard is in the writings of the canonist Cardinal Hostiensis from the 13th century AD, which were published in the 16th.