||Technology, especially artificial light at night (ALAN), often has unexpected impacts on the environment. This chapter addresses both the perception of light by various organisms and the impact of ALAN on flora and fauna. The responses to ALAN are subdivided into the effects of light intensity, color spectra, and duration and timing of illumination. The ways organisms perceive light can be as variable as the habitats they live in. ALAN often interferes with natural light information. It is rarely neutral and has significant impacts beyond human perception. For example, UV light reflection of generative plant parts or the direction of light is used by many organisms as information for foraging, finding spawning sites, or communication. Contemporary outdoor lighting often lacks sustainable planning, even though the protection of species, habitat, and human well-being could be improved by adopting simple technical measures. The increasing use of ALAN with high intensities in the blue part of the spectrum, e.g., fluorescent light and LEDs, is discussed as a critical trend. Blue light is a major circadian signal in higher vertebrates and can substantially impact the orientation of organisms such as numerous insect species. A better understanding of how various types and sources of artificial light, and how organisms perceive ALAN, will be an important step towards more sustainable lighting. Such knowledge is the basis for sustainable lighting planning and the development of solutions to protect biodiversity from the effects of outdoor lighting. Maps that describe the rapid changes in ALAN are urgently needed. In addition, measures are required to reduce the increasing use and intensity of ALAN in more remote areas as signaling thresholds in flora and fauna at night are often close to moonlight intensity and far below streetlight levels.