Near infrared spectroscopy (NIRS) is becoming a widely used research instrument to measure tissue oxygen (O2) status non-invasively. Continuous-wave spectrometers are the most commonly used devices, which provide semi-quantitative changes in oxygenated and deoxygenated hemoglobin in small blood vessels (arterioles, capillaries and venules). Refinement of NIRS hardware and the algorithms used to deconvolute the light absorption signal have improved the resolution and validity of cytochrome oxidase measurements. NIRS has been applied to measure oxygenation in a variety of tissues including muscle, brain and connective tissue, and more recently it has been used in the clinical setting to assess circulatory and metabolic abnormalities. Quantitative measures of blood flow are also possible using NIRS and a light-absorbing tracer, which can be applied to evaluate circulatory responses to exercise along with the assessment of tissue O2 saturation. The venular O2 saturation can be estimated with NIRS by applying venous occlusion and measuring changes in oxygenated vs. total hemoglobin. These various measurements provide the opportunity to evaluate several important metabolic and circulatory patterns in very localized regions of tissue and may be fruitful in the study of occupational syndromes and a variety of diseases.