This book deals with the mechanism of signal transduction in vertebrate and invertebrate photoreceptors. It contains contributions on the structure and function of rhodopsin or other G-coupled receptors, on the regulation of second messengers by enzyme cascade, the role of Ca2+ in light adaptation, control of ionic channels in photoreceptor cells. Some key points: Rhodopsin - structure and function; transducin and phosphodiesterase; arrestin and kinase; cGMP-gated channel; role of Ca2+ in photoreceptors; transduction in invertebrates; eyes of halobacteria.