The benefits of living and training at high altitude (HiHi) for an improved sea-level performance have been questioned because controlled studies have shown contradictory results. HiHi increases red blood cell mass (RCM), but training in hypoxia may be either an inadequate (low-intensity) or even harmful (to heart, muscle, and brain) stimulus. Recent studies indicate that the best approach to attain the benefits and overcome the problems of altitude training is to sleep at a natural or simulated moderate altitude and train at low altitude or sea level (HiLo). HiLo training increases RCM, as well as sea-level VO(2max) and performance (at least in responders), if certain prerequisites are fulfilled. The minimum dose seems to be more than 12 hours per day for over 3 weeks at an altitude or simulated altitude of 2100 to 2500 m. The effects of exposure to hypoxia seem to persist for a short period during the subsequent training or racing in normoxia.