This article explores the question of “Left fascism,” which emerged in relation to discussions around the Student Movement in the German Federal Republic in the crucial decade between 1967-1977. The term was originally coined by Jürgen Habermas in a lecture entitled “The Phantom Revolution and its Children” in which he suggests that the extreme voluntarism of the students could not but be characterized as “Left fascist.” Such a characterization becomes the basis for a vitally important exchange of letters between Herbert Marcuse and Theodor W. Adorno from January to August of 1969 on the relation between theory and praxis. After first sketching Adorno's conception of the “authoritarian personality,” with the help of Sándor Ferenczi's concept of the “identification with the aggressor,” the article proceeds to examine the exchange of the letters between Adorno and Marcuse, illustrating Adorno's changed orientation: that “fascism” or “authoritarianism” maybe either left or right. Finally, some conclusions are drawn about the authoritarian tendencies of the contemporary Left.