Studies of cancer among farm workers are difficult to conduct and interpret given the unique nature of this occupational group. The transitory nature of the work, high levels of poverty, and lack of legal documentation make epidemiologic studies difficult to accomplish. Nevertheless, this workforce in the United States, which numbers as much as 3 million persons, is a high risk population due to exposures to numerous toxic substances, including excessive sunlight, heat, dangerous machinery, fumes, fertilizers, dust, and pesticides. We summarize characteristics of farm workers (i.e., demographics, health care) from the National Agricultural Workers Survey (NAWS) and the California Agricultural Workers Survey (CAWS) and present findings from a series of studies conducted among farm workers in California. The epidemiology literature was reviewed and methods for a unique farm worker union-based epidemiologic study are presented. Farm workers in California and the rest of the United States, many of whom are seasonal and migrant workers are at elevated risk for numerous forms of cancer compared to the general population and specific pesticides may be associated with this altered risk. Elevated risks have been found for lymphomas and prostate, brain, leukemia, cervix, and stomach cancers.