The subject of obligations must be determined, possible and permitted by law. Obligations can be created by the will of legal entities or against the will of legal entities. The subject of obligation law is the rights and obligations arising from the obligation relationship, whereby the creditor has the right to demand certain giving, doing, not doing or suffering, and the debtor, on the other hand, has the obligation to do so. Subjects of obligations are natural and legal persons between whom there is a debtor-creditor relationship. Obligatory rights are relative, i.e. they have effect only among subjects, participants in a given obligation relationship, and there are a few exceptions to this rule.

The Law of Obligations has not undergone significant changes since its inception until today. The Law on Obligatory Relations includes primarily contracts as sources of obligations, i.e. the reasons for the emergence of obligations, but also the causing of damage, acquisition without basis, management without warrant, unilateral declaration of will. Other laws introduce other sources of obligations, i.e. other prescribed facts that create obligations, such as marriage.