Part I described the physiological factors defining the limits of static apnea, while Part II examined performance in dynamic distance swimming. This paper reviews the factors determining performance in the depth disciplines, where hydrostatic pressure is added to the stressors associated with apneaduration and physical work. Apneic duration is essential for performance in all disciplines, and is prolonged by any means that increases gas storage or tolerance to asphyxia, or that reduces metabolic rate. For underwater distance swimming, the main challenge is to restrict metabolism despite the work of swimming, and to redirect blood flow to allow the most vital functions. Here, work economy, local tissue energy and oxygen stores, anaerobic capacity of the muscles, and possibly technical improvements will be essential for further development. In the depth disciplines, direct pressure effects causing barotrauma, the narcotic effects of gases, decompression sickness (DCS) and possibly air embolism during ascent need to be taken into account, as does the risk of hypoxia when the dive cannot be rapidly interrupted before the surface is reached again. While in most deep divers apneic duration is not the main limitation thus far, greater depths may call for exceptionally long apneas and slower ascents to avoid DCS. Narcotic effects may also affect the ultimate depth limit, which elite divers predict to he around 156 metres' sea water. for constant weight with tins. To reach these depths, serious physiological challenges have to be met, technical developments are likely to be needed and safety procedures must be developed concomitantly.