Altitude and heat acclimatization have been used by athletes for centuries in preparation for competitions held in those specific environments. Since the 1990s high altitude has also been used as a stimulus by a large body of athletes in an attempt to bolster exercise performance for competitions held also at sea level. The scientific data supporting such strategies range from “no benefit” to “highly relevant”. In this presentation focus will be devoted to potential sources of variations in response to altitude exposure that may explain this divergence. Within the last 5-10 years also heat training has become a popular training method among especially cyclist with the intend to increase exercise performance in a thermo-neutral environment. At present there seems to be less variance in response hereto as compared to hypoxic strategies. These studies will be summarized. As a final point, data on the direct comparison of altitude and heat training will be presented.