Mysticete whales produce a variety of low-frequency sounds. These range from the highly labile song of the humpback whale (Megaptera novaeangliae) to the infrasonic sounds of the blue (Balaenoptera musculus) and fin (Balaenoptera physalus) whales. Biological functions of these sounds remain largely untested, the most common being that sounds are a male reproductive display for social communication. Recent access to fixed arrays of bottom-mounted hydrophones and towed arrays has provided a novel mechanism for documenting low-frequency whale sounds throughout large ocean areas in relation to season, bathymetric features, and ocean conditions. Ocean sound propagation conditions impose severe constraints and offer dramatic opportunities for acoustic transmission for whales. The bioacoustic data in combination with sound propagation models provide novel insights into the possible biological functions for low-frequency whale sounds, and offer testable predictions. Improved understanding of low-frequency whale bioacoustics is critical for knowing the potential impact of human-made low-frequency sounds on whales, and offers insights into improved signal design. Biological functions will be presented in conjunction with empirical evidence to support, for example, that blue and fin whales navigate using reverberation and bistatics, and humpback males should prefer deep rather than shallow water singing habitat.