We are now witnessing a resurgence of theories of development and carcinogenesis in which the environment is again being accepted as a major player in phenotype determination. Perturbations in the fetal environment predispose an individual to disease that only becomes apparent in adulthood. For example, gestational exposure to diethylstilbestrol resulted in clear cell carcinoma of the vagina and breast cancer. In this review the effects of the endocrine disruptor bisphenol-A (BPA) on mammary development and tumorigenesis in rodents is used as a paradigmatic example of how altered prenatal mammary development may lead to breast cancer in humans who are also widely exposed to it through plastic goods, food and drink packaging, and thermal paper receipts. Changes in the stroma and its extracellular matrix led to altered ductal morphogenesis. Additionally, gestational and lactational exposure to BPA increased the sensitivity of rats and mice to mammotropic hormones during puberty and beyond, thus suggesting a plausible explanation for the increased incidence of breast cancer.