This review describes the normal biology and physiology of the mammary gland in macaques, including the typical histologic appearance across the life span (development, reproductive maturity, lactation, and senescence). The molecular events regulating breast morphogenesis are described, as well as systemic and local hormonal regulators of mammary gland proliferation, differentiation, and function. Similarities and differences to the human breast are described. Regulatory events are illuminated by discussion of genetically modified mouse models. Tissue response markers, including immunohistochemical markers of proliferation and other hormonally induced changes and studies to date, regarding the effects of exogenous hormones, are briefly summarized. In general, estrogens stimulate progesterone receptor expression and proliferation in the mammary gland, and combinations of estrogens and progestogens cause greater proliferation than estrogens alone. Evaluation of novel chemical agents in macaques requires careful evaluation of age and hormonal context to avoid the confounding effects of mammary gland development, past reproductive history, and other influences on mammary gland morphology. The expression of proliferation markers and progesterone receptors may be used as biomarkers to measure chemically induced hormonal effects.