New developments and applications of autonomous Passive Acoustic Monitoring (PAM) technology in polar regions have come at time of increased interest in the Arctic and Antarctic due to predictions of global climate change. Information gained with autonomous PAM systems has provided new information on polar pinniped communication, mating systems, distribution, and the relationships between these species and their environment. Although new discoveries continue to be made, there is still much that remains to be learned about these species. This chapter is organized into a review of species specifi c information known prior to 2000, case studies describing new knowledge gained through the use of autonomous PAM systems since 2000, and future projection on how autonomous PAM systems can be used to address and fi ll data gaps related to polar pinnipeds.