The son of a famous French physiologist, JEAN MAYER came to the United States before the war to pursue his studies; but the war called him home, and as a Gaullist he fought for five years with the Free French forces. He got a Ph.D. at Yale Medical School in 1948, a D.Sc. from the Sorbonne, and in 1950 joined the faculty of the Department of Nutrition at the Harvard School of Public Health, where his studies on obesity have attracted wide attention. This is the last of three articles by Dr. Mayer.