The severe attenuation and scattering properties of water necessitate the use of artificial illumination in many underwater viewing and photographic situations. This has been of increasing importance in recent years as men descend deeper into the oceans where natural light is nonexistent. These same properties also impose severe limitations on the use of artificial light in water. In addition to these optical properties, constraints are also imposed by the physical properties of water, particularely pressure. There are, however, elements of control that can be exercised in the design and application of underwater light sources, which will permit some degree of optimization. A few principlesof under-water illumination have been evolved which can be applied to optimize artificial illumination for a variety of applications.