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Introduction: There have been few well controlled altitude and hypoxic training studies to date. This thesis investigated the effects of altitude and (sham controlled) intermittent hypoxic training (IHT) on exercise capacity, and the associated physiological adaptations. Methods: Chapter 3 investigated how living and training at 2320 m or at sea level affected total haemoglobin mass (tHb) and race performance in highly trained swimmers. Chapter 4 investigated how IHT or normoxic training affected cardiopulmonary variables and the incremental exercise limit of tolerance (T-Lim), in highly trained runners. Chapter 5 investigated how single-legged IHT or normoxic training affected phosphorus-31 nuclear magnetic resonance spectroscopy assessed muscle energetics. Results: In Chapter 3, tHb increased significantly more after altitude (+0.6 ± 0.4 g•kg-1, or +4.4 ± 3.2%) than after sea level (+0.03 ± 0.1 g•kg-1, or +0.3 ± 1.0%), but the changes in swimming performances were not different between groups, and there were no correlations between tHb and performance changes. In Chapter 4, submaximal heart rate in normoxia decreased significantly more after IHT than after normoxic training (-5 ± 5 vs. -1 ± 5 b∙min-1), and submaximal "V" ̇O2 in hypoxia significantly decreased, only after IHT. T-Lim in hypoxia significantly increased post-IHT, but there were no between group differences. In Chapter 5, the phosphocreatine recovery time constant was speeded significantly more in the IHT compared to the normoxic trained leg, when tested in hypoxia (-25 ± 8% vs. -13 ± 6%), but not in normoxia (-16 ± 15% vs. -9 ± 10%). Conclusions: Altitude training likely increases tHb, but this is not necessarily associated with improved athletic performance. IHT may induce other non-haematological adaptations; potentially an enhanced skeletal muscle oxidative capacity, but evidence for exercise capacity gains is lacking. The precise underlying causes for these adaptations require further investigation, as does any translation to athletic performance.