John Desmond Bernal (1901-1971) was undoubtedly the most important of the "Western" scientists who, during the twentieth century, accepted the Marxist view of social development. He did more than "accept" it: he tried to sketch the whole history of science from a Marxist viewpoint; he wrote a number of articles explicitly expounding his view of the relation of Marxism to science; and from his student days he played an active role in Communist politics. He has been criticised: during his lifetime, for too readily accepting official Soviet policy, whether relating to society or to science; since his death, for having been too ready to hope that his vision of the use of science for human ends could be implemented by capitalist societies; and at all times, for an allegedly simplistic faith in science as the salvation of mankind.