Exposure of divers to intense noise in water is increasing, yet no general hearing-conservation standard for such exposures exists. This paper summarizes three theories of underwater hearing, the tympanic, bone-conduction, and dual path theories, and reviews empirical data in order to identify some requirements such a standard must meet. Among problems considered are hearing sensitivity in water, the frequency and dynamic ranges of the water-immersed ear, and non-auditory effects of underwater sound. It was concluded that no well-developed theoretical basis exists for extrapolating current hearing-conservation standards for air-borne noise to the underwater situation; the existing empirical evidence, except within the frequency range of 1.5-3.5 kc/s, is too scant to predict what levels of underwater noise would be safe for divers; the empirical data on underwater hearing suggest that the frequency range covered by an appropriate standard must be much broader, for both low and high frequencies, than is the case in air; in order to establish an appropriate standard, further research is required on the dynamic range of the ear in water; problems must be solved with shifts in the resonance frequency of the ear consequent to the introduction of helium-oxygen mixtures or other exotic gases and pressures during dry-helmet diving or during simulated underwater excursions in dry hyperbaric chambers; underwater noise exposure may involve hazards to other body systems than the ear; and some noise exposure conditions may interfere with job performance of divers.