This book, now in a thoroughly revised second edition, offers a comprehensive review of the rapidly growing field of optogenetics, in which light-sensing proteins are genetically engineered into cells in order to acquire information on cellular physiology in optical form or to enable control of specific network in the brain upon activation by light. Light-sensing proteins of various living organisms are now available to be exogenously expressed in neurons and other target cells both in vivo and in vitro. Cellular functions can thus be manipulated or probed by light. The new edition documents fully the extensive progress since publication of the first edition to provide an