The influence of light on the lives of living organisms is all-pervasive, affecting movement, vision, behavior, and physiological activity. This book is a biophysically grounded comparative survey of how animals detect light and perceive their surroundings. Included are discussions of photoreceptors, light emitters, and eyes. The book focuses in particular on the kinds of optical systems that have evolved, beginning with unicellular organisms that detect and respond to light through to more advanced and complex designs for imaging. The relevance of these studies extends beyond biology, since these findings can be used to help develop photoreceptor energy conversion and information systems, and optical imaging devices with a wide range of everyday applications. The book will appeal to biophysicists, photobiologists, bioengineers, neuroscientists, and all researchers working in the area of vision and visual optics.