Underwater sound recording of animals uses specialized techniques to obtain faithful copies of sounds produced by animals during their normal activities underwater. Techniques have to be unobtrusive as well as nondisturbing to avoid changing the animal behaviors. The first scientific recording of underwater sounds from a marine mammal at sea was by William E. Schevill and Barbara Lawrence in 1948. Although the equipment has changed considerably since then, the techniques, approaches to animals and environmental impedimenta have remained essentially the same. However, the frequency and dynamic ranges of underwater sounds can easily exceed terrestrial sounds, so the selection of suitable equipment is critical. The elements of a useful system for bioacoustic recording of marine animals include the hydrophone, impedance transformer/preamplifier, cable, signal amplifier, recorder and sound monitor. The important criteria for each of these is discussed, along with directional listening systems, and the need for calibrations to verify the performance of the entire underwater recording system. For each situation, the ideal system is the one with the best compromise of interactive component to record that particular sound spectrum.