The breast is one of two prominences located on the upper ventral region of a primate's torso. Both females and males develop breasts from the same embryological tissues.

In females, it serves as the mammary gland, which produces and secretes milk to feed infants. Subcutaneous fat covers and envelops a network of ducts that converge on the nipple, and these tissues give the breast its size and shape. At the ends of the ducts are lobules, or clusters of alveoli, where milk is produced and stored in response to hormonal signals. During pregnancy, the breast responds to a complex interaction of hormones, including estrogens, progesterone, and prolactin, that mediate the completion of its development, namely lobuloalveolar maturation, in preparation of lactation and breastfeeding.

Humans are the only animals with permanent breasts. At puberty, estrogens, in conjunction with growth hormone, cause permanent breast growth in female humans. This happens only to a much lesser extent in other primates—breast development in other primates generally only occurs with pregnancy. Along with their major function in providing nutrition for infants, female breasts have social and sexual characteristics. Breasts have been featured in ancient and modern sculpture, art, and photography. They can figure prominently in the perception of a woman's body and sexual attractiveness. A number of cultures associate breasts with sexuality and tend to regard bare breasts in public as immodest or indecent. Breasts, especially the nipples, are an erogenous zone.