Bioacoustics is the study of the adaptive basis of animal sound signals: how (when, where) and why animals make sounds. Animal’s signaling behavior is a complex of structural and behavioral features that had adaptive consequences in previous generations. Bioacoustics is the investigation of these adaptive consequences. A number of taxa make very extensive use of sound signals, and in a loose sense, it may be convenient to consider them ‘acoustic animals’. The acoustic insects are the crickets and katydids, the grasshoppers and cicadas, all noteworthy sound signalers. Though the extent of the literature addressing Orthoptera and cicadas justifies the use of the term acoustic insects, in fact the incidence of sound signalling in insects, especially if one expands'sound’ to embrace'mechanical disturbances in water and solid substrata’ as well as air, is catholic. Among vertebrates, sounds are produced extensively by fish and mammals, but the frogs and birds are especially studied from an acoustic perspective. The birds are perhaps the preeminent acoustic animals. But anyone studying bats or cetaceans could find many arguments for disagreement. A final comment: we greatly underestimate the numbers of taxa that make an important use of sounds in their daily lives, particularly if we include vibration in terrestrial substrata and sound and vibration in water.