Significant advances have been made in Endocrinology relating to developmental biology, environmental physiology, chronobiology, photobiology, reproductive biology, circulatory and digestive physiology, molecular biology, metabolic physiology, clinical and medical biology, etc. Comparative points of view have also accelerated the advancement of endocrinology. This book covers various topics of endocrinology from comparative, experimental, developmental, reproductive and clinical endocrine aspects. Another important feature of this book is that more than half the chapters are described in relation to the function of melatonin and the structure of the pineal organ. These trials of this book are reasonable and timely. Melatonin physiology has been reviewed from several points of view such as antioxidant and scavenger of hydroxyl radical, circadian clock and photoperiodic gonadal response including photoreceptor system, and development of vertebrates.