Cataracts are an enormous medical, financial, and social burden. At least 300,000 to 400,000 new, visually disabling cataracts occur yearly in the United States, and 5 to 10 million worldwide. Complications of modern surgical techniques alone probably result in at least 7,000 irreversibly blind eyes annulally in the United States, and a potential 100,000 to 200,000 worldwide. These figures must be refined through well-standardized prevalence surveys and careful monitoring of indications and results of surgery. Senile cataracts probably result from many, possibly compound, obscure causes. Race, altitude, dietary habits, and deranged carbohydrate metabolism are some that have been suggested. There is little basis for the common belief that senile cataracts are more prevalent, or mature more rapidly, in diabetics. Carefully conducted case control and long-term studies are needed to assess the importance of each of these factors and to identify additional ones. The rapid increase of intraocular lens implantation raises serious questions of safety and efficacy. Implantation increases the risk of corneal and macular edema, iritis, and reoperation for lens dislocation and membrane formation, while complicating the treatment of retinal detachments and retinovascular disease. They require controlled clinical evaluation.