Although the Frankfurt School represents one of the most influential intellectual traditions of the twentieth century, its multifaceted work on modern antisemitism has so far largely been neglected. The Politics of Unreason fills this gap, providing the first systematic study of the Frankfurt School's philosophical, psychological, political, and social research and theorizing on the problem of antisemitism. Examining the full range of these critical theorists' contributions, from major studies and prominent essays to seemingly marginal pieces and aphorisms, Lars Rensmann reconstructs how the Frankfurt School, faced with the catastrophe of the genocide against the European Jews, explains forms and causes of anti-Jewish politics of hate. The book also pays special attention to research on coded and secondary antisemitism after the Holocaust, and how resentments are politically mobilized under conditions of democracy. By revisiting and rereading the Frankfurt School's original work, this book challenges several misperceptions about critical theory's research, making the case that it provides an important source to better understand the social origins and politics of antisemitism, racism, and hate speech in the modern world.