After a short introduction about biology of sea mammals and the North Atlantic, the rest of the guide is taken up with a mammal-by-mammal field guide covering all forty species of whales, dolphins, seals, walruses, and polar bears that live in the North Atlantic. Each species has a distribution map and is illustrated with photographs that show the mammals in life and illustrations that bring out the details for identification. The text gives information about behaviour, reproduction, distribution as well as providing a full description of each species.