The hearing abilities of marine mammals are addressed in this chapter. Hearing curves (audiograms) are presented for cetaceans in water and for marine carnivores in both air and water. Audiograms are presented based on whether they were collected with behavioral or electrophysiological methods, and differences in the audiograms due to the methods employed are discussed. Concepts related to hearing in noise are introduced and marine mammal adaptations for facilitating sound detection and sound source localization in noise are presented. Despite continuing research into marine mammal hearing, hearing in most marine mammal species remains untested. For species in which hearing has been measured, behavioral measurements in most species have been made in only one or a few individuals. Direct measures of baleen whale hearing are absent and will likely rely on electrophysiological methods in the smallest species. The testing of low-frequency hearing presents difficulties for electrophysiological approaches, yet low-frequency noise is generally considered of greatest concern due to its ubiquity. The development of auditory weighting functions has advanced predictions of noise impacts, but the applicability of weighting functions beyond the prediction of noise-induced hearing loss in marine mammals remains to be fully explored. Additional research into basic hearing abilities and auditory physiology will be essential to improving our understanding, modeling, and assessing the impacts of noise on marine mammal behavior and physiology.