A breast implant is a technique used in cosmetic surgery to increase the size of a woman's (or a man's) breasts or to reconstruct the breast (for example, after a mastectomy). There are two main types of breast implants:
The health hazards of breast implants have been debated greatly in recent years. Some people believe that breast implants cause such illnesses as autoimmune disease, although bothe the AMA and FDA have found there to be no evidence of this.  However, documented problems with breast implants include rupture, deflation, infection, scarring and hardening of the implants. 
According to the American Society of Plastic Surgeons, breast augmentation is the most commonly performed cosmetic surgical procedure on women in the United States. In 2002, 236,888 women in the United States underwent breast augmentation.  According to the National Institute for Women, one in four silicone implant recipients must undergo surgery, within 5 years, to correct implant problems.
See also: breast reconstruction