The founding of the Communist Party of Yugoslavia and the beginnings of communist ideology in Yugoslavia can be attributed to the influence of the October Revolution and individuals who spread communist ideas. Since the CPY operated in two phases, legal and illegal, this paper deals with both, but with a special emphasis on the illegal phase, which lasted longer. By synthesizing the relevant academic literature and using the paper Proleter as a historical source, the thesis investigates the illegal activities of the CPY in the interwar period with an emphasis on the period of publication of this newspaper. The thesis through the chapters thematically presents the circumstances that influenced the development of the CPY, an overview of political and social changes in Europe in the 1930s, growing fascism, the Great Purge, conflicts that led to a new great war. The evolution of the CPY in the interwar period was caused by its adaptation to the new changes taking place within the party and the state, but also in the world and the Comintern, which controls the main determinants of its ideological struggle.