This thesis determined the metabolic and transcriptional responses to high-intensity interval training (HIIT) in human skeletal muscle. The overall hypotheses were that (1) six weeks of HIIT (10x4 min cycling intervals | 90% VO2peak with 2min rest in between, ~1 hr/d, 3d/wk) would increase substrate transport and oxidative capacities, (2) mitochondrial responses would be greater if 60% O2 was inspired during HIIT, (3) mitochondrial enzymatic increases would occur within two weeks, (4) these responses would be preceded by initial increases in the content of putative transcriptional regulators of mitochondrial biogenesis and (5) HIIT would increase the ratio of mitochondrial fusion to fission proteins in healthy recreationally active young men and women. The metabolic responses following six weeks of HIIT included increases in five key mitochondrial enzymes, VO2peak and fat oxidation at 60% pre-training VO2peak. HIIT also increased time to exhaustion by ~2-fold at 90% pre-training VO2peak with attenuated glycogenolysis, lactate accumulation, PCr utilization and the increases in ADPf and AMPf and did not change PDH activation. HIIT increased transport proteins for glucose (GLUT4), lactate (MCT1,4) and free fatty acids (FAT/CD36, FABPpm) and glycogen content. Increases in three mitochondrial enzymes were not greater after inspiring 60% O2 during HIIT (6 weeks) despite an 8% higher training power output for the same training heart rate. Mitochondrial adaptations following HIIT in room air occurred rapidly, with increases in CS and [beta]-HAD observed after three HIIT sessions. This was preceded by increases in PGC-1[alpha], PPAR[alpha] and PPAR[gamma] contents after one session. PGC-1[beta] and PPAR[beta]/[delta] contents increased by three and five sessions respectively but no change occurred in NRF-2 and tFAM. Mitochondrial fission proteins (Fis-1 and DRP-1) increased after three sessions whereas the fusion protein MFN-1 increased after seven sessions. These investigations demonstrate that HIIT increases the capacities for carbohydrate and fat transportation and oxidation in human skeletal muscle despite a reduced volume of training per week. Potential augmentation of the stimulus with hyperoxia does not amplify the mitochondrial responses. Nevertheless, mitochondrial increases occur rapidly and are likely amplified by early increases in transcriptional proteins. Alterations in mitochondrial morphology may also be prominent early in HIIT.