We identified the occupations that employ California women and a list of chemicals of concern for breast cancer. We evaluated the likelihood of on-the-job exposure to the categories of chemicals by occupation among formally and informally employed women. We selected 145 occupations representing more than 6.6 million women (85% of California working women), along with an additional sixteen occupations for informal workers only. We organized 1012 chemicals (including mammary gland carcinogens, developmental toxicants, and endocrine-disrupting chemicals) into twenty-five categories. More than 80 percent of occupations investigated had possible or probable exposure to at least one category of chemicals. This is the first categorization of occupational exposure to chemicals of concern for breast cancer among California working women. Our investigation revealed significant data gaps, which could be improved by policy changes resulting in enhanced collection of data on occupation and chemical exposure.