Neither the etiology nor the natural history of AMD or cataract is known. Epidemiologic studies suggest that a number of risk factors may be associated with AMD and cataract, but the strength of the evidence in support of these hypotheses varies. Possibly associated with AMD are personal characteristics, such as age, race, height, family history, and strength of hand grip; ocular characteristics, such as hyperopia and color of iris; and cardiovascular diseases, smoking, lung infections, and chemical exposures. Clinical and laboratory studies suggest the following factors may be associated with progression of AMD: drusen type, choroidal vascular diseases, and photic injury.