This book is actually a product of efforts of many people, not only of the authors. W ide investigations of marine mammals began in Russia (that time, in the former Soviet Union) in the 1960s when a few teams of enthusiasts founded in captivity experimental stations intended for keeping dolphins and seals and for performing experimental studies of these fascinating animals. It was a time when attention of many people throughout the world was attracted to dolphins and other marine mammals due to appearance of oceanariums and dolphinariums, which demonstrated unique capabilities of these animals. So scientists in many countries concentrated on studies of them. There was much to learn about the morphology, physiology, and psychology of marine mammals, and investigators spending their time and efforts on studies in this field were rewarded by a number of surprising findings. The authors of this book represent one of such research teams focused on the neuro- and sensory physiology of marine mammals. A few decades of studies naturally resulted in the idea to summarize in a book both the results of these studies and a large body of data in adjacent fields. Our goal was to synthesize the many research findings and the present knowledge on sensory capabilities and mechanisms of sensory systems of aquatic mammals. We realize, however, that the appearance of this book was made possible due to the help and assistance of many colleagues.