This intellectual biography recovers the legacy of Karl Popper (1902-1994), the progressive, cosmopolitan, Viennese socialist who combated fascism, revolutionized the philosophy of science, and envisioned the Open Society. Malachi Hacohen draws a compelling portrait of the philosopher, the assimilated Jewish intelligentsia, and the vanished culture of Red Vienna, which was decimated by Nazism. Seeking to rescue Popper from his postwar conservative and anticommunist reputation, Hacohen restores his works to their original Central European contexts and, at the same time, shows that they have urgent messages for contemporary politics and philosophy.