NEW FINDINGS What is the topic of this review? We review recent work relating to exercise-induced alterations in mitochondrial structure and function. What advances does it highlight? Training mitochondrial volume density increases due to 1) an increase cross sectional area and 2) longitudinal growth. Specific respiratory alterations appear dependent on exercise training intensity. Low-moderate endurance training primarily improves the capacity for fat oxidation whereas high-intensity interval training (HIT) improves global respiratory capacity. The latter includes maximal state 3 mass-specific respiration, which is the strongest individual measure predictive of endurance performance. This highlights the importance of training specificity in endurance athletes. Mitochondrial volume density (MitoVD ) is composed of two distinct mitochondrial subpopulations- intermyofibrillar mitochondria (MitoIMF ) and subsarcolemmal mitochondria (MitoSS ). With exercise training, MitoVD may increase by up to 40% and is, for the most part, related to an increase in MitoIMF . Exercise-induced adaptations in mitochondrial function depend on the intensity of training and appear to be explained predominately by an increased expression of mitochondrial enzymes that facilitate aerobic metabolism. Although mitochondrial content often increases with training, it seems that mitochondrial adaptations are not needed to facilitate maximal oxygen uptake, whereas such adaptations are of greater importance for endurance capacity.