Optogenetics is a fast-growing field concerning the invention, and use, of molecules that are genetically expressed in cells, and then either report on cellular physiology in optical form, or enable control of specific pathways in cells when actuated by light. This book reviews the current state, as well as the underlying principles and future directions, of a diversity of optogenetic tools of importance for observing and controlling cellular signaling and physiology. These tools are already revolutionizing neuroscience, and are starting to have impact on fields ranging from metabolism to development to cardiology. The book contains a dozen chapters written by world experts on these topics, half focusing on the optogenetic molecular tools themselves, and half on the genetic and hardware approaches that make them powerfully useful in intact tissues and organisms.