Among the transparent tissues of the eye, the lens is a rather bradytrophic compartment having a relatively xenobiotic metabolism. It is composed of specialized proteins, whose optical properties are dependent on the fine arrangement of their three-dimensional structure and hydration. Protein-bound SH-groups of the crystallins are protected against oxidation and cross-linking by high concentrations of reduced glutathione. Their molecular composition as well tertiary and quaternary structures provide a high spatial and timely stability (heat-shock proteins) especially of the larger crystallins, who are able to absorbe radiation energy (shortwave visible light, ultraviolet and infrared radiation) over longer time periods without basicly changing their optical qualities. This provides considerable protective function also for the activity of various enzymes of the carbohydrate metabolism. The glucose metabolic pathway is functioning rather anaerobicly with low energetic efficiency, nevertheless it has to provide the metabolic energy for protein synthesis, transport and membrane synthesis. In addition, the syncytial metabolic function of the denucleated fiber cells has to be maintained by the epithelium and the small group of fiber cells, which still have their metabolic machinery. This results in a steep inside-out metabolic gradient, which is complicated by the fact that the lens has a kind of repair system shutting of damaged groups of fiber cells (wedge- or sectorial cataracts). All epithelial cells of the lens are subjected to light and radiation stress leading to alterations of the genetic code. Because defective cells cannot be extruded, these are either degraded (apoptosis, necrosis), or they are moved to the posterior capsular area, where they contribute to the formation of posterior subcapsular cataracts. Ageing generally reduces the metabolic efficiency of the lens thus increasing its succeptability to noxious factors. Ageing provides the grounds where cataract noxae can act and interact to induce the formation of a variety of cataracts, many of them being associated with high protein-related light scattering and discoloration.