This thesis examines the biogeography of world pinnipeds, a unique group of marine mammals that have adapted to marine foraging while maintaining terrestrial (land or ice) habitat links. Comparative analyses of species range sizes controlled for phylogenetic relationships using a multi-gene supertree with divergence dates estimated using fossil calibrations. Adaptations to aquatic mating and especially sea ice parturition have influenced range size distribution, and ranges are larger than those of terrestrially mating and/or pupping species. Small range size is endangering for many taxa, and most at risk pinnipeds are terrestrial species with small ranges. Ancestral state reconstructions suggest that pinnipeds had a long association with sea ice, an adaptation that would have allowed early seals to expand into novel habitats and increase their distribution. Range sizes exhibit a strong Rapoport effect (positive relationship between range size and latitude) at the global scale, even after controlling for phylogeny and body size allometry. A latitudinal gradient in species diversity cannot explain the Rapoport effect for global pinniped ranges, as diversity is highest at mid-latitudes in both hemispheres. These regions are characterized by marginal ice zones and variable climates, supporting a mix of pagophilic and temperate species. The climatic variability hypothesis also did not explain the Rapoport effect. Variability is bimodal, and annual sea surface temperature (SST) variability does explain diversity patterns. Range size has a significant negative relationship with annual mean SST, and the largest ranges are found in areas with low mean SST. Temperature responses are possibly related to thermoregulation, sea ice availability, and ecological relationships with other large marine predators. These results agree with other studies and suggest that ocean temperature, and not productivity, drives marine species richness patterns. Future research needs include studies of physiological tolerances, interactions with sharks as predators and competitors, and the role of climate and sea ice in speciation and evolution. A better understanding of distribution and diversity patterns, and the role of the environment in shaping these patterns, will improve conservation efforts, and studies on the role of SST and sea ice are particularly important given current warming trends and declines in ice extent.