Cataract is the most important pathologic condition of the crystalline lens and one of the most frequent causes of blindness in the world. Cataracts may be the consequence of a multitude of developmental, chemical, or physical disturbances of the lens and may occur ‘spontaneously’ in the course of lens aging. Although surgical removal of the opaque lens does lead to satisfactory results in many patients, it is necessary that research into the biochemical and biological properties of the lens will clear up the mechanisms of opacification. In recent years, the combined efforts of clinicians, biochemists, biologists, and physicists have made it possible to elucidate some special types of lens opacity. These results have helped in the development of preventive measures and in some instances in the evolution of conservative therapy of initial cataracts. However, the causes and pathogenetic mechanisms of the most frequent forms of lens opacity, i.e. the senile cataracts, are to date still unknown. The different clinical types of senile opacity are discussed and present-day theories on causative factors and biochemical development are reviewed. Difficulties in senile cataract research are discussed and the need for a satisfactory classification of human cataract lenses is pointed out. The enormous importance of interdisciplinary basic research on the lens for a further elucidation of cataract development is stressed.