Description: The Arctic and Antarctic share a strong seasonality of light and temperature extremes that create similar selective pressures for animals living in these two polar and associated sub polar regions. There is significant evidence that Arctic ice conditions are rapidly deteriorating due to global climate change and evidence that Antarctic ice is also changing. Sub polar regions experience a seasonal ice cover with an added dynamic range of environmental conditions often exceeding that at the poles. In polar and sub polar waters, the presence of sea ice dominates the physical environment for an extended period of time. This strongly influences pinniped distribution, reproductive strategies, foraging ecology, and acoustic behavior. In both polar regions, seals have distinctly different underwater vocal repertoires associated with breeding and an airborne repertoire associated with mother and pup communication. In this chapter, a comparative approach is taken to relate the underwater acoustic behavior of polar pinnipeds (phocids and walrus) to their ecology and aspects of their sea ice habitat. Understanding the commonalities and differences in the spatial, spectral, and temporal characteristics of vocalizations from species with comparable biologies relative to local sea ice conditions may provide insights into the acoustic ecology of pinnipeds in polar habitats. Acoustic ecology describes the interaction between an animal and its environment as mediated through sound and is determined by the species' behavioral ecology, but also by biotic and abiotic factors of the environment. Insight into acoustic ecology may therefore provide information on the potential direct and indirect impacts of polar climate change on pinnipeds. Loss of sea ice will likely affect polar pinniped distribution and breeding activities; both of which can be detected through long-term passive acoustic monitoring. One of the secondary effects of sea ice loss is the impact to the polar acoustic soundscape (i.e., the acoustic environment). The communication system of polar pinnipeds, which permeates critical life functions like breeding, evolved in an acoustic environment dominated by ambient noise levels associated with seasonal ice cover. Changes in ice dynamics will likely be accompanied by shifts in habitat use by both marine animals and humans which will lead to a corresponding change in the acoustic soundscape. The combined impacts of these effects is unprecedented, but the knowledge gained from a comparison of pinniped acoustic ecology between the Arctic and the Antarctic will lead to a better understanding of the relationship between ice seals and their changing environment.