Light is both an energy source and a carrier of information. Changes in light intensity, direction, quality, spectral composition, and the metronomic predictability of the Earths' light-dark cycles carry vital information about the environment. Photoreceptors have evolved that sense these characteristics of light and trigger cellular signaling processes that control physiology, development, and behavior and enable organisms to develop a sense of space and time. In plants and fungi the photoreceptive rhodopsins, light oxygen voltage proteins, phytochromes and cryptochromes sense changes in and facilitate the adaptation to the light environment. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.