The great prestige of the Nobel Prizes has come virtually to define physics in the public mind. An examination of the nominations and the selections in the first 37 years of Nobel awards shows that those in charge of the selections went through a process of development as they refined their conceptions of the kind of work appropriate to be rewarded. Analyses are presented of the 1800 nominations made by 448 nominators for 213 scientists, of whom 44 received physics prizes between 1901 and 1937, 10 more who were awarded Nobel Prizes in physics after 1937, and 12 in chemistry, although nominated also in physics.