Book excerpt: 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 up-to-date overview of the physical, chemical, and biological properties of light-sensing proteins and their application in biological systems, particularly in neuroscience but also in medicine and the optical sciences. Underlying principles are explained and detailed information provided on a wide range of optogenetic tools for the observation and control of cellular signaling and physiology, gene targeting technologies, and optical methods for biological applications. In presenting the current status of optogenetics and emerging directions, this milestone publication will be a “must read” for all involved in research in any way related to optogenetics.