Recent reports on the ultraviolet radiation emitted by tubular fluorescent lamps, coupled with the fashion for higher levels of illumination and large expanses of window glass have aroused the concern of museum officials regarding the damaging effect of natural and artificial light. The first part of the article includes a review of the principles of photochemistry. The second part is concerned with the effect of light on varnishes and paint vehicles, as well as on paper, and refers to recent technical publications on the subject. The significance of temperature, humidity and the wavelength of radiation are discussed as well as the exposure in terms of footcandle hours necessary to cause a given degree of change. Among the effects of ultraviolet and the short wavelengths of visible radiation are the erosion of the surfaces of varnishes and paint vehicles, resulting in shrinkage, yellowing and bleaching. Moreover, varnishes tend to become more resistant to the action of solvents through oxidation or the formation of a crosslinked type of molecular structure, thus making it more difficult to remove them when desired. The increase in the refractive index of linseed oil, resulting in the appearance of pentimenti, may be hastened by exposure to light. Although visible radiation is not absorbed to any degree by cellulose, the glue, rosin and other constituents of ordinary paper are able to absorb the violet and blue radiation, producing yellowing and embrittlement of the paper.