As large-bodied predators, pinnipeds (seals, fur seals, sea lions, and walruses) play essential roles in the structure and function of marine ecosystems through consumer-prey interactions. Solid knowledge of their foraging ecology and trophic niche is critical to investigating changes in the structure of ecosystems over time. Diverse studies have shown that extant pinnipeds display divergent foraging strategies and habitat preferences. However, it is not well understood what mechanisms underlie these different strategies, nor how they have evolved and affected community structure. In this dissertation, I use stable isotope and morphometric analyses on museum specimens to examine the foraging ecology, niche segregation, and ecomorphology of extant pinnipeds and to uncover how extinct pinnipeds partitioned their ancient communities and how these dynamics compare to modern ones.In this dissertation, I show that sympatric otariids across the North Pacific and the Southern Hemisphere display distinct foraging modes, consistent with energetic tradeoffs associated with their body size disparities. Fur seals predominantly feed offshore and on pelagic food webs, whereas sympatric sea lions rely on nearshore and benthic resources, resulting in comparable niche segregation patterns across regions and stressing the driving role of body size in otariids' foraging behavior and niche partitioning. I also conducted an exhaustive examination of sympatric otariids from the eastern North Pacific, the most diverse otariid community in the world. I demonstrate that California sea lions (Zalophus californianus), Steller sea lions (Eumetopias jubatus), northern fur seals (Callorhinus ursinus), and Guadalupe fur seals (Arctocephalus townsendi) display significant differences in their size, feeding morphology, and foraging. Body size and feeding morphology are not related to foraging at the intraspecific level in a consistent way. However, when species and sex identity are excluded, foraging is significantly related to body size and feeding morphology emphasizing their underlying role in niche segregation at the community level. In a closer examination of California sea lions, I show that male sea lions from the North Pacific have significantly increased rather than decreased their body size over the last five decades of population recovery. These surprising results demonstrate that body size decrease is not a general response to increased resource competition during the recovery of marine mammals and reveal how density-dependent sexual and natural selection might affect body size, feeding morphology, and in turn the foraging ecology of marine predators. Little is known about how pinniped foraging ecology and niche partitioning have evolved over geologic time, and additional quantitative and comparative studies are needed to test palaeoecological hypothesis based on comparative morphology. Before addressing this subject, I first examined the modes and trends of the pinniped fossil record. My co-authored work demonstrates that the pinniped fossil record is adequate and does not show significant biases, validating its suitability for paleoecological investigations. Then, by conducting stable isotope analysis on fossil tooth enamel of fossil pinnipeds from Southern California and western North Atlantic, I reveal that fossil pinniped assemblages had foraging patterns analogous to those described in modern communities (i.e., with nearshore and offshore foraging modes). These results suggest that these foraging modes were acquired in pinniped communities early in their evolutionary history, hinting that niche partitioning has contributed to the structure of pinniped communities over time.