People are exposed to a myriad of chemicals in their daily life. Modest toxicity information exists for only some of these chemicals, including their ability to alter breast development and disease risk. Yet, breast cancer remains one of the leading causes of death among women. In animal models, early life exposures to low doses of chemicals, particularly endocrine disrupting substances, increase the risk for breast cancer by affecting mammary development and lifetime susceptibility to cancer. Chemical regulations currently under development as part of California’s Green Chemistry Initiative will require new decision-making tools for identifying and prioritizing chemical hazards. Over the past year, an expert panel convened by the University of California, Berkeley has examined the biological pathways governing breast carcinogenesis and the toxicity tests that can improve scientist’s ability to detect chemicals likely to affect breast development and carcinogenesis. The panel recommends improvements to chemical toxicity screens to increase their relevance for human breast cancer. Panel recommendations are based on currently available toxicity tests and biomolecular and pathology studies. They define a testing scheme for identifying chemicals that may affect breast development and cancer susceptibility, reproductive harm, and other cancers. Identifying such environmental factors will inform effective regulation of chemical hazards and improve breast cancer prevention efforts.