Cardiorespiratory fitness is indicated by the body's ability to transport oxygen from the surrounding air to the contracting muscle, and the muscle's capacity to use oxygen. Cardiorespiratory fitness relies on the effective integration of the cardiovascular pulmonary, hematologic, and skeletal muscle systems. Maximal oxygen uptake is considered the best measure of cardiorespiratory fitness. A low level of maximal oxygen uptake is associated with a markedly increased cardiovascular and all-cause mortality both in healthy subjects and cardiac patients. Randomized, controlled trials are still needed, however, to further clarify the value of the assessment of cardiorespiratory fitness in clinical decision-making.