Cataract surgery, also called lens replacement surgery, is the removal of the natural lens of the eye (also called "crystalline lens") that has developed an opacification, which is referred to as a cataract, and usually its replacement with an artificial intraocular lens.

Metabolic changes of the crystalline lens fibers over time lead to the development of the cataract, causing impairment or loss of vision. Some infants are born with congenital cataracts, and certain environmental factors may also lead to cataract formation. Early symptoms may include strong glare from lights and small light sources at night, and reduced visual acuity at low light levels.

During cataract surgery, the cloudy natural lens is removed, either by emulsification in place or by cutting it out. An artificial intraocular lens (IOL) is usually implanted in its place to restore useful focus. Cataract surgery is generally performed by an ophthalmologist in an out-patient setting at a surgical center or hospital. Local anaesthesia is normally used, and the procedure is usually quick and causes little or no pain and minor discomfort to the patient. Recovery sufficient for most daily activities usually takes place in a matter of days, and full recovery about a month.

Well over 90% of operations are successful in restoring useful vision, with a low complication rate. Day care, high volume, minimally invasive, small incision phacoemulsification with quick post-operative recovery has become the standard of care in cataract surgery all over the developed world. Manual small incision cataract surgery (MSICS) is popular in the developing world, as it is considerably more economical in time, capital equipment and consumables, while providing comparable results. Both procedures are low risk for serious complications.