In this paper, the development of extramarital union in Croatian legislation will be through the historical development in the European area up to what is important to us for the purposes of this paper, the Croatian legal system. Therefore, in the first part of the work, extramarital union is presented form the Roman law until the seventies of the twentieth century. The definiton of marriage, family, cohabitation goes through various systems and finding an optimal answer that would satisfy all legal systems is a big challenge. In the second part of the paper, we are presented with all the important European regulations that protect unions that were not created in the traditional way, by marriage. These regulations are important to us because they have beed adopted in the territory of the Republic of Croatia, where they applied and readily provide answers to questions regarding the rights and obligations of extramarital unions. The main of all regulations together is always the protection of the weaker party. In the end, all regulations turn to the protection of the individual and their rights. Such developments led to equating cohabitation with marriage. The paper analyzes relevant legal sources and court practice, which is very important to us, especially when it comes to weaker parties.