Technical Abstract: Animal vision, the flowering of plants, and light-seeking movement of microbes are all examples of light-controlled behavior mediated by photosensory receptors. Upon illumination, these specialized pigment-containing proteins trigger a physiological response. Understanding their molecular function is the basis for the dissection of higher-level processes, such as biological rhythms and the sleep-wake cycle, plant development, and microbial ecological organization. This first comprehensive resource on photosensory receptors from archaea, bacteria, plants and animals compiles the data on all known classes of photoreceptors, creating a must-have reference for students and researchers for many years to come. Each of the molecular mechanisms of light perception is systematically covered, with emphasis given to the primary photoreceptor pigments, their photochemistry and regulation. As such, this is the first resource to compare the different types of photosensory receptors found in different organisms, as well as their role in cellular physiology. This book chapter provides an up to date in-depth analysis of phytochrome signaling research.