Light in a museum serves several purposes. In the first place, it renders the exhibits visible and readable. Accentuating light guides the visitors to where the designer has wanted them to go. It is needed for orientation, and in the storerooms for carrying out maintenance, making inventories, inspection, and investigation. When daylight is admitted, artificial light is added as accentuating light, but to what extent? A short survey of the properties and the physical characteristics of light, including ultraviolet and infrared radiation, is given. Recommendations include: avoid great contrasts that have a dramatizing effect and involve a high local intensity, use a low intensity background lighting system, reduce daylight, make a flexible light plan. Lamps should not be installed in showcases because of heat development. Avoid reflections and light sources annoying to visitors.