This chapter explores the origins, development and applications of Aron’s theory of totalitarianism from the 1930s to the 1950s. It begins by discussing how Aron’s earliest theorisations of totalitarianism and political religion emerged from critical dialogues with the works of Élie Halévy and Carl Schmitt such that by the eve of the Second World War Aron had arrived at an understanding of totalitarianism as a pathology of modern democracy. The chapter then considers how Aron’s theory of totalitarianism developed with the onset of the Cold War and how this impacted on his understanding of the meaning of modern democracy as a constitutional, pluralist, multi-party regime. It concludes with a discussion of Aron’s theory of totalitarianism in relation to that of Hannah Arendt, explaining how and why Aron came to de-emphasise notions of totalitarianism and secular religion in his work following the death of Stalin in 1953.