J. B. S. Haldane (1892-1964) was one of the most brilliant of British scientists - and one of the most controversial. A trail-blazing geneticist and physiologist, who used himself as his own guinea-pig, he was also a highly successful populariser of science, a dedicated Marxist, and a devotee of Hindu culture. His private life was often tempestuous: early in his career he was sacked from his Cambridge post after being cited in a divorce case - but reinstated on appeal; and his relations with scientific colleagues and the political establishment were normally acrimonious. Haldane's most important scientific research, on the mathematical basis of evolutionary theory, was done at University College London. Towards the end of his life he founded the Genetics and Biometry Laboratory at Bhubaneswar in India having become an Indian citizen in 1960. In writing this definitive biography, Ronald Clark was able to draw upon Haldane's private papers, as well as the reminiscences of the great man's friends (and enemies). Mr. Clark has written extensively on scientists and the application of science to modern life. His books include major biographies of Einstein and Freud.