Breast cancer is the most common non-skin cancer among women in this country. Breast cancer risk is significantly influenced by genetics, but over 70% of the women that are diagnosed have noninherited or sporadic cancer. The risk of breast cancer is thought to be modified by lifestyle and environment. Exposures to certain chemicals and hormone-mimicking or endocrine-disrupting compounds (EDCs) are suspected of contributing to increased breast cancer incidence as well as precocious puberty in the United States. Studies of EDC effects in rodents indicate that multiple toxicants can alter mammary gland development, with or without changing other markers of puberty. EDCs can cause transient and persistent effects on mammary gland development depending on dose, exposure parameters, and whether exposure was during critical periods of gland growth or differentiation. Adverse effects from these abnormal developmental patterns include the presence of carcinogen-sensitive structures in greater numbers or for longer periods in the gland and inhibited functional differentiation leading to malnutrition or increased mortality of their offspring. Developmental toxicants of the mammary gland could lead to an increase in the incidence of mammary tumors if they alter circulating or tissue-localized hormone levels, gland receptor expression patterns, hormone transport, or metabolism that results in altered response to endogenous hormones or growth factors. Environmental disruptors of rodent mammary gland development must be identified for informed decisions in epidemiological studies aimed at identification of environmental factors contributing to breast cancer risk, altered breast development during puberty, or inability to produce sufficient breast milk.