Adaptation to living or training in hypoxic environments (altitude training) continues to gain interest from sport scientists and endurance athletes. Here we present the first meta-analytic review of the effects on performance and related physiological measures following adaptation to six protocols of natural or artificial hypoxia: live-high train-high (LHTH), live-high train-low (LHTL), artificial LHTL with daily exposure to long (8-18 hours) continuous, brief (1.5-5 hours) continuous or brief (<1.5 hours) intermittent periods of hypoxia, and artificial live-low train-high (LLTH). The 51 qualifying studies provided 11-33 estimates for effects on power output with each protocol and up to 20 estimates for effects on maximal oxygen uptake (VO2max) and other potential mediators. The meta-analytic random-effect models included covariates to adjust for and estimate moderating effects of study characteristics such as altitude level and days of exposure. Poor reporting of inferential statistics limited the weighting factor in the models to sample size. Probabilistic inferences were derived using a smallest worthwhile effect on performance of 1%. Substantial enhancement of maximal endurance power output in controlled studies of subelite athletes was very likely with artificial brief intermittent LHTL (2.6%; 90% confidence limits+/-1.2%), likely with LHTL (4.2%; +/-2.9%), possible with artificial long continuous LHTL (1.4; +/-2.0%), but unclear with LHTH (0.9; +/-3.4%), artificial brief continuous LHTL (0.7%; +/-2.5%) and LLTH (0.9%; +/-2.4%). In elite athletes, enhancement was possible with natural LHTL (4.0%; +/-3.7%), but unclear with other protocols. There was evidence that these effects were mediated at least partly by substantial placebo, nocebo and training-camp effects with some protocols. Enhancing protocols by appropriate manipulation of study characteristics produced clear effects with all protocols (3.5-6.8%) in subelite athletes, but only with LHTH (5.2%) and LHTL (4.3%) in elite athletes. For VO2max, increases were very likely with LHTH (4.3%; +/-2.6%) in subelite athletes, whereas in elite athletes a 'reduction' was possible with LHTH (-1.5%; +/-2.0%); changes with other protocols were unclear. Effects on erythropoietic and other physiological mediators provided little additional insight into mechanisms. In summary, natural LHTL currently provides the best protocol for enhancing endurance performance in elite and subelite athletes, while some artificial protocols are effective in subelite athletes. Likely mediators include VO2max and the placebo, nocebo and training-camp effects. Modification of the protocols presents the possibility of further enhancements, which should be the focus of future research.