The effect of light on animal tissues is ambivalent. Light is necessary for many functions, e.g. for vision and, as in the flagellate halobacterium, to gain energy. But light is potentially dangerous: it is capable of destroying cells or their components by photooxidation, especially in the presence of sensitizing pigments such as haems and cytochromes, which are ubiquitous in aerobic cells. Several different examples are discussed to show how a compromise is achieved in animal tissues that for functional reasons receive high exposure to light. Carotenoid pigments, present in many eyes and photoreceptors, seem especially suited to protect against the deleterious effects of light because they absorb the dangerous short wavelength part of the light spectrum. In plant tissue, carotenoids are also well known to be capable of 'quenching' photoexcited states of sensitizing pigments and of oxygen, a function that they might have also in animal tissue. A consequence of the considerations is that whenever animal tissues are exposed to higher than usual light levels and/or oxygen pressures cellular damage might occur. Examples are discussed; strategies to circumvent the deleterious effects by photooxidation follow directly from the arguments.