Pacta sunt servanda is one of the most important principles of contract law today. It expresses the obligation to respect contracts and fulfill the obligations arising from them. Hugo Grotius is considered the founder of the understanding of pacta sunt servanda. He believed that trust should be established between individuals in society, thus security that the obligations from the contract will really be fulfilled and respected. Roman law also enforced the sanctity of contracts. The first model of this understanding was typical for ancient and medieval Roman law. Only selected types of contracts (nominate contractscontractus) that were concluded with the application of certain norms provided the parties with full protection. Later, this group of agreements expanded and began to include unnamed contracts and certain types of informal agreements (pactas). It is important to note that the maxim pacta sunt servanda was not absolute even in Roman law, i.e. there were exceptions when one could deviate from the duty to fulfill obligations from the contract. Today, the Law on Obligations, specifically Article 9 of ZOO, prescribes the duty to fulfill obligations. The legislator deliberately prescribes the duty to fulfill obligations, thus determining that there is a duty to fulfill obligations arising from the contract as well as extra-contractual ones. Following the Roman legal tradition, Croatian legislation prescribes several institutes of deviation from the duty to fulfill obligations, such as changed circumstances, excessive damage, force majeure, greenback contract, termination of contract, etc.