Karl Popper (1902–1994) is one of this century’s most influential philosophers, but his life in fin-de siècle and interwar Vienna, and his exile in New Zealand during World War II, have so far remained shrouded in mystery. This intellectual biography recovers the legacy of the young Popper; the progressive, cosmopolitan, Viennese socialist who combated fascism, revolutionized the philosophy of science, and envisioned the Open Society. Malachi Hacohen delves into his archives (as well as the archives of his colleagues) and draws a compelling portrait of the philosopher, the assimilated Jewish intelligentsia, and the vanished culture of Red Vienna, which was decimated by Nazism. Hacohen’s adventurous biography restores Popper’s works to their original Central European contexts and, at the same time, shows that they have urgent messages for contemporary politics and philosophy.