Oxidative metabolism is the dominant source of energy for skeletal muscle. Near–infrared spectroscopy allows the non–invasive measurement of local oxygenation, blood flow and oxygen consumption. Although several muscle studies have been made using various near–infrared optical techniques, it is still difficult to interpret the local muscle metabolism properly. The main findings of near–infrared spectroscopy muscle studies in human physiology and clinical medicine are summarized. The advantages and problems of near–infrared spectroscopy measurements, in resting and exercising skeletal muscles studies, are discussed through some representative examples.