Lens opacities in old patients are usually classified as "senile cataracts". Since there are very distinct morphologic types of opacities it is evident that we have to deal with different types of triggers in discussing the cataractogenesis. A great variety of risk factors might act as triggers for the development of opacities in aged people, and there is no doubt that among them age changes of lens metabolism are the most important. These age induced changes of lens metabolism do not only influence the energy level of lens metabolism (energy concept of lens transparency), the trigger may as well be located within the protein synthesizing system or the processes responsible for maintaining normal protein conformation, further, an exogenous intoxication may also occur. Lack of normal human lenses of all ages suited for metabolic research and an undefined classification for instance, coloration of the cataractous lens) makes biochemical research with respect to the pathogenesis of senile cataracts almost impossible. Animal models (naphthalene cataract, tryptophane-deficiency, X-irradiation) suited for the purpose may well support the basic research of this subject, even if we have to assume certain species differences of lens metabolism.