Increasing noise levels are reshaping marine soundscapes, prompting growing concern about how marine mammals can detect biologically important acoustic cues in a louder ocean. This chapter briefly summarizes available knowledge of auditory masking in acoustically reliant pinnipeds (sea lions, walruses, and seals) and reviews new hearing measurements that expand understanding of their frequency selectivity and susceptibility to noise. Recent behavioral studies provide directly comparable measurements of critical ratios and critical bandwidths for representatives of the three pinniped families. Results demonstrate broad similarities between sea lions and walruses in terms of masking susceptibility and frequency tuning, while seals show enhanced specialization for hearing in noise. Additional psychophysical measurements conducted with one highly trained California sea lion in an acoustically controlled environment examine the onset of masking at low noise levels and show that noise becomes audible before it begins to degrade signal detection. Together with prior laboratory studies of hearing, these experiments continue to refine auditory parameters relevant to predicting masking in natural environments.