The thesis examines the legal mechanisms for transferring obligations and rights in Roman law and compares them with the modern Croatian legal system, as regulated by the Obligations Act. The paper provides a detailed analysis of key legal institutions such as cession (transfer of claims), expromissio (transfer of debt), novation (novatio), assumption of debt, and assumption of performance. Roman law developed key legal tools for the transfer of rights and obligations, ensuring the stability of legal relationships. These institutions, with necessary adjustments, have survived to this day. The modern croatian Obligations act retains the fundamental principles of Roman law but has adapted them to the needs of the modern economy. Through an analysis of cession and expromissio in Roman law and a comparison with today's legal solutions, the thesis demonstrates how Roman legal concepts remain relevant for understanding and regulating substitution of subjects in obligatory relationships. Changes in the subjects of obligatory relationships enable flexibility and adaptability of legal relations to new conditions, which is a crucial component for both roman and contemporary croatian law.