This paper is both a review of pertinent data in the literature and a presentation of data derived from investigations at the Mellon Institute. The authors conclude that visible light in the 420-510 nm range tends to bleach paper containing lignin, shortwave ultraviolet radiation in the 360-390 nm range tends to cause darkening of papers containing light in, and bleaching and darkening occur at the same time when both types of radiation are present. That thermally induced darkening can occur after a paper is exposed to light is a possibility which the authors feel must be investigated carefully in the immediate future.