Cataract is the major cause of blindness worldwide. It is a greater problem in third world countries than in the West and several attempts have been made to explain the excess in these countries. This paper provides an overview of the literature especially on studies designed to identify risk factors for cataract. There is an association between poverty and cataract and, more specifically, between cataract and a history of severe diarrhoea-dehydration. Recent results from a case-control-led study of cataract in Oxford are also presented with the quantitation of risks associated with a number of factors including diarrhoea, renal failure and diabetes. In this study an apparently protective effect of aspirin, paracetamol and similar drugs was observed. This protective effect applies to the risk associated with diabetes.