The concept of totalitarianism emerged between the two world wars in twentieth-century Europe to become a central concept of Cold War social science designed to highlight similarities between the Nazi and Soviet regimes and implicitly to contrast these forms of dictatorship with liberal democracy. While in the 1960s and 1970s many critics challenged the concept’s Cold War uses as an ideology of “the West,” the idea of totalitarianism and later “post-totalitarianism” played important roles in East Central Europe, where they helped dissident intellectuals, academics, and activists both to understand and to challenge Soviet-style communism. The concept of “totalitarianism” remains heavily contested. But whatever one thinks about the concept’s social scientific validity, there can be no doubt that it played a crucial role in both the scholarship of communism and the public intellectual debates about the possibilities of post-communism. Aviezer Tucker’s The Legacies of Totalitarianism: A Theoretical Framework (Cambridge 2015) addresses many of these issues, and so we have invited a range of political scientists to comment on the book and the broader theme denoted by its title.