This text presents findings in the study of the sensory functions of aquatic mammals, with an emphasis on hearing, vision, and somatic senses. The book opens with a discussion of the anatomy and mechanics of the cetacean middle and inner ear and demonstrates what makes it particularly sensitive to high frequencies; it then focuses on vision in aquatic mammals: specifically, those with centrally-symmetric eye optics with a quasi-spherical lens and hemispheric eyecup. Finally, it concludes with a comparative analysis of sensory systems of aquatic mammals and terrestrial mammals.