Photoreceptors are nerve end organs or receptors that are sensitive to light. In the retina of the eye, the outer limbs of rods and cones make up the photoreceptor layer. This book presents topical research in the study of the physiology, types and abnormalities of photoreceptors. Topics include the mechanisms for tuning a photoreceptor's spectral sensitivity; foveal microstructure observed by optical coherence tomography; the caudal photoreceptor in the crayfish and quantum catches and photoreceptors.