Analyzing structure and function in specialized ears can produce new insights into fundamental hearing mechanisms and lead to technological advances. Research into dolphin echolocation is a classic example. Recently, however, concerns over anthropogenic sounds in the oceans pushed us to develop a broader knowledge of marine mammal hearing, and, in the last five years, hearing research on marine mammals expanded considerably. The resulting data on their hearing, ear anatomy, and vocalizations suggest that marine mammal ears are more diverse and complex than previously expected, with acoustic capabilities spanning infra to ultrasonic ranges. Seals are amphibious hearers with middle and inner ears similar to land carnivores, while the ears of whales are strikingly different and are adapted exclusively to hearing underwater. Consistent with high sound speeds in water, specialized fats, not air-filled canals, conduct sound to the ear, and both middle and inner ears are located well outside the skull. Vestibular components are reduced, consistent with cervical fusion related to hydrodynamic body shapes; but their cochlear components, particularly the auditory fibers, are hypertrophied. Neural hypertrophy may be adaptive for high background noise, but may also be related to exceptional signal processing mechanisms in both infra and ultrasonic whales.