The first four decades of the 20th century were glorious years for science, especially physics. Our view of the physical world changed forever with the emergence of quantum mechanics and Einstein?s formulation of the theory of relativity. India too contributed significantly to this scientific revolution with the discoveries made by S N Bose, C V Raman and M N Saha, all in the space of about a decade. Kariamanikkam Srinivasa Krishnan (1898- 1961) belonged to the same illustrious group. He was perhaps the only Indian physicist of his generation who was equally adept in theory and experiment. Besides a life of excellence in science, Krishnan?s destiny led him to be an able science policy maker and administrator. He was also a great teacher, a humanist and a scholar of Sanskrit, Tamil literature and philosophy. This biography, besides being a detailed and meticulously documented account of Krishnan?s life and his scientific work, is also an exciting account of the history of Indian science of the period. The source material of this work, most of which are being used for the first time, comes from the private papers of K S Krishnan that had remained in the custody of his family.