The term photopollution is proposed for artificial light having adverse effects on wildlife. The differences between natural and artificial light are discussed in relation to the concepts of orientation, disorientation, misorientation and abnormal orientation. The ways in which optic orientation systems are attuned to natural illumination conditions are analysed, and it is shown why they therefore may fail to cope with artificial light. It is concluded that for many nocturnally active animals a natural light-field between sunset and sunrise is a requirement for survival. A review is given of data on a) bird kills at man-made lighted obstacles, and b) the interference of artificial light with nest site selection by female sea turtles and water-finding by hatchlings at nesting beaches. Conventional remedies against the hazards of photopollution are critically reviewed and new ones are suggested. It is emphasized that measures should aim not only at reducing threats to a species or population but also at preventing suffering in individual animals.