North American river otters (Lontra canadensis; referred to simply as river otters from here on) are widely distributed across North America with the exception of the desert Southwest. Extirpated over much of their original range, reintroduction programs in 21 states have been extensive and river otters now occupy at least portions of their historic range in every state within the continental United States except New Mexico. Over most of their range they can be found in freshwater habitats ranging from alpine lakes to rivers, streams and swamps. From California to Alaska, they also occupy a nearshore marine habitat where they primarily feed on marine invertebrates and fishes and play and important ecological role moving marine nitrogen and phosphorus into nearshore vegetation. Their dependence on freshwater for drinking, however, limits them to marine habitats close land where freshwater is available. With river otter populations recovering, veterinarians have an increased chance to encounter them in both wildlife management and rehabilitation settings. Clinicians working with river otters should be familiar with their clinical anatomy and diseases.