There is growing international concern about the effects of human-generated sound on fish and other aquatic organisms. However, because of a striking paucity of well-designed and controlled experimental data, very little is actually known about the effects of these sounds on fish. Findings suggest that human-generated sounds, even from very high intensity sources, might have no effect in some cases or might result in effects that range from small and temporary shifts in behavior all the way to immediate death. At this point, however, it is nearly impossible to extrapolate from results with one sound source, one fish species, or even fish of one size to other sources, species, or fish sizes. The present paper briefly discusses the potential effects of sound on fish, describes some of the more recent well-controlled experimental studies, and points out areas for future study that will be needed before a real understanding of the effects of sound on fish can be developed.