A cataract is a clouding of the crystalline lens (clear tissue behind the pupil) that causes a gradual loss of vision. When the lens becomes opaque, light is unable to reach the retina, resulting in blurred vision. Cataracts are variously classified according to cause (aging, first trimester rubella infection, myotonic dystrophy, Down syndrome, trauma, radiation), location (nuclear, cortical, subcapsular), severity (immature, mature, hypermature), and age of onset (congenital, juvenile, adult, age-related). Approximately one million cataract surgeries are performed each year in the United States, while 5 to 10 million individuals annually are disabled as a result of cataracts.

Aging is the most common cause of cataracts; in fact, most patients over 50 years of age demonstrate cataractous changes. However cataracts may also be congenital, hereditary, or associated with systemic, metabolic, or ocular diseases, trauma, radiation, maternal infection during pregnancy, electrical injuries, and drug use.