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 alrea...