In this master's thesis, the institutes of validation and conversion of invalid legal transactions are thoroughly analyzed within the context of Roman and Croatian law. The aim is to provide a comprehensive understanding of these legal concepts through a detailed analysis of legal transactions in general, their modes of conclusion, the diversity of types, and particularly the aspects of their validity and invalidity. Furthermore, the thesis will elucidate the concepts of nullity and voidability, with a particular focus on their interpretation in accordance with the provisions of the Law on Obligations of the Republic of Croatia. Research into the differences between voidability and nullity began only in the 19th century, and thanks to the pandectistic doctrine, the concept of invalidity of legal transactions has been shaped into a model that predominates today and is accepted within the legal circle of continental Europe. Systematic observation of the Roman and Croatian legal systems will encompass the historical development of the institutes of validation and conversion and provide insight into the influence of Roman legal arrangements on contemporary law, especially in the field of private law in the Croatian legal system. Through the analysis of practical examples from the court decisions of Croatian courts, the thesis will strive to highlight the application of Roman concepts of validation and conversion in contemporary Croatian law.