The subject of optogenetics is comprehensively covered in this book, including physical, chemical, and biological topics of light-sensing proteins and their application in biological systems, particularly in neuroscience and medicine and the related opto-electronics. Optogenetics is a new technology that combines genetics and optics. It enables one to manipulate or measure the function of identified cells or neurons in a tissue by light with an accuracy in the range of milliseconds, even in a freely moving animal. Optogenetics has already become a powerful tool for revealing the neural mechanisms underlying behavior and analyzing various physiological phenomena. It is also expected to become useful for treating neural dysfunctions such as Parkinson disease and for the development of a brain-machine interface. This book should be read by any scientist or student performing research in any way related to optogenetics. As a milestone publication on optogenetics, this book will serve as a compass for any researcher, from beginners to experts, to explore this uncharted world.