Phocid (or earless or true) seals are ecologically diverse, occupying habitats from the tropics to the poles in marine and freshwater and feeding on anything from tiny zooplankton to other marine mammals. There are 18 species of phocid seals, the smallest species (ringed seal) is more than 20 times smaller than the largest (southern elephant seal), with marked sexual dimorphism present in some species. This book examines the behavior, ecology, and physiology that allow phocid seals to inhabit such a wide range of habitats. The book is composed of 16 chapters written by 37 authors from 8 countries. The book first describes the general patterns of phocid behavior, followed by descriptions of what is known about well-studied species. We have taken a holistic approach, focusing not only on the behaviors themselves but also on the factors that constrain the expression of behavior and the proximate mechanisms driving behavior. In many cases, the chapters represent collaborations between well-established researchers and early-mid career individuals who bring new perspectives to help carry the field of phocid behavioral ecology well into the future.