Artificial light is not only one of humanity's greatest achievements and of considerable importance to the way we work and live. Artificial light also influences the biological rhythm of day and night and thus the overall structure of the natural balance. In addition to the intended effects, the increasing use of artificial outdoor lighting also entails a number of undesirable side effects referred to as light pollution. TAB's policy brief summarizes the scientific state of knowledge with regard to the extent and trends of this light pollution and explores further potential needs for action.