Internal migration is a growing social phenomenon of today's America: a third of the United States population live in a different state from the one in which they have been born. This, however, has been a constant aspect of the American experience. The author of the present essay analyzes in an historical perspective the growth of California from 1900–1920 under the impact of the westward movement. The social, economic and political implications of the California development are the main features of this paper.