The sense of hearing in animals has been studied quantitatively using a variety of behavioral techniques for the past 50 years. The database now existing [R. Fay, Hearing in Vertebrates: A Psychophysics Databook (Hill-Fay Associates, Winnetka, IL, 1988)] includes data for 180 vertebrate species and allows several generalizations to be made regarding the relations between human and animal hearing, the methodologies useful in animal hearing studies, and the hearing processes common to all vertebrates. Although the frequency range of hearing varies by many octaves among vertebrates, absolute sensitivity at best frequency is quite similar (± 10 dB re: 0.0002 dyn cm−2). The sound discrimination performance often, but not always, exceeds the performance of animal listeners. There are no clear phylogenetic trends toward more acute discrimination abilities in primates, and very few examples of specializations for enhanced sound discrimination abilities among vertebrates (echolocators may be exceptions). There is no single conditioning or psychophysical method that clearly gives lower or more valid thresholds. Data from the literature will be presented to illustrate these and other general conclusions about animal hearing.