The deployment of active sonar by the U.S. Navy and its potential impacts on marine mammals has been an ongoing issue of intense debate; regulatory, legislative, and judicial activity; and international concern. Some peacetime use of military sonar has been regulated under the Marine Mammal Protection Act (MMPA) and other statutes due to concerns that active military sonars are operated at frequencies used by some cetaceans (i.e., whales, porpoises, and dolphins), and their highintensity sound pulses may travel long distances in the ocean. There is also concern that sonar transmissions of sufficiently high intensity might physically damage the hearing in cetaceans or cause them to modify their behavior in ways that are detrimental. Although mid-frequency sonar has been implicated in several beaked whale strandings, there is scientific uncertainty surrounding the totality of the effects active sonar transmissions may have on marine mammals.