In order to understand fish hearing, it is critical to have a basic understanding of underwater sound. The inner ear of fishes consists of three semicircular canals for detection of angular motion of the body and three otolith organs, the saccule, lagena, and utricle, that respond to linear acceleration and gravity as well as sound. Hearing results from the fish’s body moving in the sound field with the water, while the otolith moves with a different amplitude and phase due to its very different density. By being a particle motion detector, the fish ear can only detect a limited frequency range, and sensitivity to sounds at any particular frequency is also limited. A perusal of G. Retzius and more recent work shows remarkable diversity in the morphology of the ears of fishes. In effect, concerns about potential effects of anthropogenic sounds on fishes becomes a “driving force” for studies of fish hearing.