One of the functions of the human lens is filter light between 300-400 nm from reaching the retina. The lens is therefore continually under photooxidative stress. In the young lens the primary absorbing species is the O-beta glucoside of 3-hydroxykynurenine (3-HKG) which has a maximum at 365 nm. Photophysical studies have demonstrated that absorptions by this compound in the short term are relatively benign to the lens, but in the long term can lead to the photochemical loss of 3-HKG with the concomitant yellowing of lens proteins. It will be proposed that part of this yellowing is due to the photochemically induced attachment of 3-HKG to lens proteins. The yellowing of lens proteins leads to a drastic increase in the number of photons absorbed by the lens. This, along with the age-related losses of antioxidants such as GSH will increase the photooxidative stress on the lens. Considering the foregoing and various epidemiological, model and biochemical studies, it can be concluded that light is most likely one of the causative factors in cataractogenesis.