This chapter introduces hard evidence into a debate too frequently carried out on the level of polemics and conjecture. Mexican immigrants have become prominent in the California economy, first as agricultural field laborers and later in urban labor markets. The chapter reviews developments in seven California labor markets that reveal the diverse roles played by Mexican workers. The life-cycle patterns of the migratory social networks and the individuals within them help to explain the role of Mexicans in US labor markets. During the 1960s and early 1970s, progressive citrus packers in Ventura County, California, made efforts to stabilize a previously migrant Mexican labor force. Ventura County’s citrus industry represents a clear example of how employers can cut the fringe benefits of settled immigrants and reduce their labor costs by tapping into immature immigrant networks. The example of San Diego County tomato growers demonstrates that employers can abandon a well-established immigrant network for an entirely new and easily exploited group.