The development of the human breast is dependent on the presence of ovarian steroids. The basic secretory units--the alveoli--continue to respond to steroids throughout the reproductive years. Lactogenesis is triggered by a rapid and drastic fall in progesterone at delivery and maintained by prolactin while the actual expulsion of milk from the breast depends on oxytocin. The composition of milk is very variable but is adequate to provide the sole source of nutrients for up to the first 6 months of life. Lactation suppresses ovarian activity probably through a disturbance in the pulsatile pattern of LH secretion but the degree of suppression depends on infant feeding patterns and perhaps on maternal nutritional status. Breastfeeding therefore confers a degree of protection against pregnancy but some artificial methods of contraception are appropriate for use during lactation. It is still not clear whether breastfeeding protects significantly against breast cancer.