On July 1, 2013, the Republic of Croatia experienced great success by becoming a full member of the European union, a supranational institution whose motto is „United in diversity“. Through this paper, the history of the establishment of the European union and the history of relations between the Republic of Croatia and the European union will be analysed. After analysing the history, we will look back at the course of the negotiation process with an emphasis on the negotiation process of the Republic of Croatia. Furthermore, special attention will be paid to the adaptation of legislation where the Republic of Croatia had to adapt it to the acquis of the European union through various and important reforms. After painstaking negotiations and adaptation of the legislation, we will look at the process of accepting membership and the process of joining the European union itself. Furthermore, through the analysis of the structure of the European union, the Croatian representatives will be presented. At the very end, the advantages and challenges that the Republic of Croatia has to deal with as a member of the European union will be presented.