A breast implant is a prosthesis used to enlarge the size of a woman's breasts (known as breast augmentation, breast enlargement, mammoplasty enlargement, augmentation mammoplasty or the common slang term boob job) for cosmetic reasons; to reconstruct the breast (e.g. after a mastectomy; or to correct genetic deformities), or as an aspect of male-to-female sex reassignment surgery. According to the American Society of Plastic Surgeons, breast augmentation is the most commonly performed cosmetic surgical procedure in the United States. In 2006, 329,000 breast augmentation procedures were performed in the U.S.

There are two primary types of breast implants: saline-filled and silicone-gel-filled implants. Saline implants have a silicone elastomer shell filled with sterile saline liquid. Silicone gel implants have a silicone shell filled with a viscous silicone gel. There have been several alternative types of breast implants developed, such as polypropylene string or soy oil, but these are uncommon and not recommended.