This is a brief overview of physiological reactions, limitations and pathophysiological mechanisms associated with human breath-hold diving. Breath-hold duration and ability to withstand compression at depth are the two main challenges which have been overcome to an amazing degree as evidenced by the current world records in breath-hold duration at 10:12 min and depth of 214 m. The quest for even further performance enhancements continues among competitive breath-hold divers, even if absolute physiological limits are being approached as indicated by findings of pulmonary edema and alveolar haemorrhage post-dive. However, a remarkable, and so far poorly understood, variation in individual disposition for such problems exists. Mortality, connected with breath-hold diving is primarily concentrated to less well trained recreational divers and competitive spear fishermen who fall victim to hypoxia. Particularly vulnerable are probably also individuals with pre existing cardiac problems and possibly, essentially healthy divers who may have suffered severe alternobaric vertigo as a complication to inadequate pressure equilibration of the middle ears. The specific topics discussed include the diving response and its expression by the cardiovascular system which exhibits hypertension, bradycardia, oxygen conservation, arrhythmias and contraction of the spleen. The respiratory system is challenged by compression of the lungs with barotrauma of descent, intrapulmonary hemorrhage, edema and the effects of glossopharyngeal insufflation and exsufflation. Various mechanisms associated with hypoxia and loss of consciousness are discussed, including hyperventilation, ascent blackout, fasting and excessive post exercise oxygen consumption. The potential for high nitrogen pressure in the lungs to cause decompression sickness and nitrogen narcosis is also illuminated.