The surgical removal of the entire breast, usually to treat serious breast disease; such as, breast cancer: "In mastectomy, doctors remove the entire breast, a major chest muscle, and lymph glands in the armpit in an effort to make sure that the cancer has been eliminated."

There are four general types of mastectomy

1. Subcutaneous mastectomy when the entire breast is removed but the nipple and areola (the pigmented circle around the nipple) remain in place.
2. Total (or simple) mastectomy or the removal of the whole breast, but not the lymph nodes under the arm (axillary nodes).
3. Modified radical mastectomy or the removal of the whole breast and most of the lymph nodes under the arm (axillary dissection).
4. Radical mastectomy which is the removal of the chest wall muscles (pectorals) in addition to the breast and axillary lymph nodes. For many years, this operation was considered the standard for women with breast cancer, but it is rarely used today.

While the patient is anesthetized (unconscious and pain-free), an incision is made into the breast. The breast tissue is removed from the overlying skin and the underlying muscle.

When an axillary dissection is done, it is typically via the same incision.

One or two small plastic drains are usually left in place to prevent fluid from collecting in the space where the breast tissue used to be.

It is possible to reconstruct the breast (with artificial implants or native tissue) at the same operation (immediate reconstruction) or at a later date, after other necessary treatments are given (delayed reconstruction).