The epidemiologic evidence is not yet clear or consistent as to all the major risk factors for breast cancer. This chapter considers the relations between breast cancer and breastfeeding, and explores the influence of breastfeeding and other reproductive practices in the subsequent development of breast cancer in women. It explains the effects of breastfeeding or lack of breastfeeding in infancy and the long-term risk of breast cancer in adulthood. Evolutionary biology would suggest that for the preponderance of human evolutionary history, most women spent much of their adult lives pregnant or lactating. Historically, breast cancer has been the most frequent cancer in women and the leading cause of cancer death in women in the United States. Evidence for the role of nutrition in breast cancer relates primarily to relative dietary deficiencies of certain micronutrients, and relative dietary excesses of certain macronutrients.