The breast is a mound of glandular, fatty and fibrous tissue located over the pectoralis muscles of the chest wall and attached to these muscles by fibrous strands (Cooper's ligaments). The breast itself has no muscle tissue, which is why exercises will not build up the breasts. A layer of fat surrounds the breast glands and extends throughout the breast. This fatty tissue gives the breast a soft consistency and gentle, flowing contour. The actual breast is composed of fat, glands with the capacity for milk production when stimulated by special hormones, blood vessels, milk ducts to transfer the milk from the glands to the nipples and sensory nerves that give feeling to the breast. These nerves extend upward from the muscle layer through the breast and are highly sensitive, especially in the regions of the nipple and areola, which accounts for the sexual responsiveness of some women's breasts.

Because the breast is made up of tissues with different textures, it may not have a smooth surface and often feels lumpy. This irregularity is especially noticeable when a woman is thin and has little breast fat to soften the contours; it becomes less obvious after menopause, when the cyclic changes and endocrine stimulation of the breast have ceased and the glandular tissue softens. Estrogen supplements after menopause can cause continued lumpiness. The breast glands drain into a collecting system of ducts that go to the base of the nipple. The ducts then extend through the nipple and open on its outer surface. In addition to serving as a channel for milk, these ducts are often the source of breast problems

The ducts end in the nipple, (which projects from the surface of the breast), and are a conduit for the milk secreted by the glands and suckled by a baby during breast-feeding. There is considerable variation in women's nipples. In some, the nipple is constantly erect; in others, it only becomes erect when stimulated by cold, physical contact or sexual activity. Still other women have inverted nipples.

Surrounding the nipple is a slightly raised circle of pigmented skin called the areola. The nipple and areola contain specialized muscle fibers that make the nipple erect and give the areola its firm texture. The areola also contains Montgomery's glands, which may appear as small, raised lumps on the surface of the areola. These glands lubricate the areola and are not symptoms of an abnormal condition.

Beneath the breast is a large muscle, the pectoralis major, which assists in arm movement; the breast rests on this muscle. Originating on the chest wall, the pectoralis major extends from deep under the breast to attach to the upper arm. It also helps form the axillary fold, created where the arm and chest wall meet. The axilla (armpit) is the depression behind this fold.

Each woman's breasts are shaped differently. Individual breast appearance is influenced by the volume of a woman's breast tissue and fat, her age, a history of previous pregnancies and lactation, her heredity, the quality and elasticity of her breast skin and the influence of hormones.