There are notably few biological data describing hearing and masking at very low frequencies for nonhuman mammals, although most natural and anthropogenic noise occurs at frequencies below a few hundred Hz. For marine mammals, auditory measurements for low-frequency sounds are particularly difficult to obtain. Physical and biological constraints have resulted in a large data gap for marine mammals with expected sensitivity to low-frequency underwater sounds. Among fully aquatic mammals, available auditory data at frequencies below 100 Hz are limited to those obtained for trained bottlenose dolphins and beluga whales in open ocean environments and for manatees tested in pools. Potential conflicts between auditory and somatic perception have been reported within these sparse data sets, complicating interpretation of the available data. More recent measurements indicate that some pinnipeds have broadly sensitive hearing that extends below 100 Hz. The low-frequency hearing of sea lions in this range is about 20 dB more sensitive than that of dolphins, while the hearing of seals is extraordinary—another 13–24 dB more sensitive than that of sea lions and extending below 40 Hz, several octaves below the nominal low-frequency hearing limit of dolphins. Sensitivity thresholds and time and frequency processing parameters recently estimated for seals now enable quantitative predictions of low-frequency hearing and masking patterns. Such carefully conducted empirical studies are advancing understanding of the auditory capabilities of some marine mammals in habitats dominated by low-frequency noise.