The article analyzes the life and work of L. Rebet, his general theoretical views on the formation and development of the nation. Considerable attention is paid to the characterization of L. Rebet’s fundamental scientific works on the theory and history of the Ukrainian nation, in particular: “The Formation of the Ukrainian Nation”; “Theory of the Nation”; “The Origin of Ukrainians, Russians, Belarusians in the Light of Modern Soviet Theories”; “The Light and Shadows of the OUN”, etc. It is shown that in his scientific concept of the formation and development of the nation, Lev Rebet examined in detail the causes of the decline, economic and cultural isolation of the Ukrainian community during the Golden Horde, analyzed the causes of the territorial fragmentation of the Ukrainian principalities and determined the importance of the Galicia-Volhynia State for the continuation of the state life of the Ukrainian people. Using historical examples, L. Rebet identified the negative impacts on the Ukrainian population under the Lithuanian and Polish occupation of Ukrainian lands. L. Rebet rightly noted that during the revival of the Ukrainian people, religion and the Orthodox faith played an important role, opposing Catholicism and polonization and becoming the bearers of Ukrainian tradition and the formation of national consciousness. It was the Ukrainian bourgeoisie, part of the petty gentry, and artisans who proved to be the bearers of culture and state traditions. However, it was only the independent Ukrainian military Cossack stratum that became the military and organizational force that quickly acquired the features of a national state, and in the era of Bohdan Khmelnytsky’s hetmanship, thanks to the military and political success of the Cossacks, one of the most powerful Ukrainian states of that time emerged, which played a significant nation-building role and for several years “crystallized the Ukrainian community, approaching a higher type - the nation”, in which the Cossacks were the main political force. The article reveals that L. Rebet in his works considered nationalism as an ideology that should be based on feelings of national dignity, self-respect and national consciousness. L. Rebet convincingly argued in his writings that the Ukrainian people have the right to political self-determination and the creation of their own state. L. Rebet believed that in order to build one’s own state, first of all, it is necessary to raise the national consciousness and culture of the people. According to L. Rebbetzin, this requires the Ukrainian people to return to their national roots, to their own history and culture, in order to preserve and establish their national identity. In his scientific concept, Lev Rebet emphasized the importance of preserving Ukrainian culture and language, and of preserving the national identity of Ukrainians. Nationalism should be democratic and social. He sought to create a national liberation organization that would unite different segments of the population and work for the benefit of the entire Ukrainian people. L. Rebet favored the creation of national statehood, but believed that the source of power in the state should be the people, not the political elite. Rebet’s views on the importance of moral virtues and cultural values, such as freedom, justice, honesty, diligence, creativity, traditions, religion, and language, are particularly relevant. Rebet believed that these values are important for Ukrainian and other nations.