Marine mammals are the aquatic organisms that depend on the ocean and other marine ecosystems for their survival. Such organisms include sea otters, seals, manatees, whales and polar bears. The adaptation of marine mammals to an aquatic lifestyle varies from species to species. Some mammals such as cetaceans and sirenians are fully aquatic. Animals such as seals and sea lions are semiaquatic. They spend a major part of their life inside water but they need to come to land to perform some specific activities such as molting, breeding and mating. Some marine mammals such as polar bears and otters are much less adapted to an aquatic lifestyle. Marine mammals have numerous anatomical and physiological features which assist them in overcoming the challenges associated with aquatic living. The topics included in this textbook on marine mammals are of utmost significance and bound to provide incredible insights to readers. Also included herein is a detailed explanation of the various concepts and theories related to this subject. This book is an essential guide for both academicians and those who wish to pursue this discipline further.