This book summarizes legal and political events related to active sonar and marine mammals. While strandings and mortality of marine mammals, primarily beaked whales, have been observed in concurrence with mid-frequency sonar operation, additional controversy has focused on the development of low-frequency active (LFA) sonar. 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 high-intensity sound pulses may travel long distances in the ocean. 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. This book also addresses the concern regarding environmental effects of ocean noise on three activities — military sonar exercises, oil and gas exploration, and commercial shipping.This is an edited, excerpted and augmented edition of a CRS publication.