In 1952, Philipp Frank came to the attention of the FBI director, J Edgar Hoover, as a possible communist sympathiser. Frank, an immigrant, had been a member of the Vienna Circle, a group of philosophers that had met from the mid-1920s to the mid-1930s. Visited by FBI agents, Frank showed them the passage in one of Lenin’s books in which Lenin savaged a paper of his. He might have added that Einstein rather liked it. Frank was one of many individuals, men and women, who played important roles in the Circle. The best known are Rudolf Carnap, Otto Neurath and Moritz Schlick, who was the leader of the group until his murder in 1936. In truth, beyond the academy, the Circle boasted no big names but, as the Frank story indicates, there were plenty on its periphery. Einstein, Wittgenstein, Russell and Popper all took their ideas seriously. A J Ayer and W V O Quine visited Vienna to meet its members; both would shape 20th-century Anglophone philosophy, drawing on their Viennese expeditions. The Circle developed a philosophical outlook known as ‘logical positivism’ or ‘logical empiricism’ – the former term is more famous; the latter fits better. This book is a readable popular history of the Circle that deftly integrates the ideas and lives of its members with the story of the Viennese