The aim of this paper is a review of the conceptualization of national interest in the theoretical assumptions of realism. A review of the literature and its analysis provides an overview of the main findings of prominent realists who shaped the national interest and created a solid foundation for future research: Hans Morgentau, Raymond Aron, and Kenneth Waltz. Consequently, the scientific goals of this paper are to summarize the scope and manner of national interest’s thematization in realist’s research and, to analyze the significance of their scientific contribution to the Science of Security. National interest remains one of the most important terms in Security Science. A prudent articulation of national interests is a key factor for the security of any state, so understanding this concept is extremely important for Security Science theorists. A realistic understanding of the national interest as a key driver, but also a determinant of the behavior of states concerning other states, is still relevant today.