The order Sirenia is the only extant group of mammals adapted to feed exclusively on aquatic plants. In view of the worldwide abundance of aquatic macrophytes and the few other large herbivores competing for this resource, it is noteworthy that Recent sirenians comprise only three genera and five species. One of these, Steller's sea cow (Hydrodamalis gigas) of the North Pacific, was exterminated by humans in the eighteenth century. Uniquely among sirenians, the Steller's sea cow was adapted to cold-temperate climates and a diet of kelp and other algae. All the living sirenians are tropical forms that feed preferentially on angiosperms, and this appears to have been the primitive condition for the order. The Indian Ocean and West Pacific tropics are today inhabited by a single species, Dugong dugon, distributed in nearshore marine waters from East Africa and the Red Sea to Japan, Micronesia, and Australia. There are four families of sirenians: Prorastomidae, Protosirenidae, Trichechidae, and Dugongidae. This chapter describes the systematic paleontology of Sirenia.