Near-infrared spectroscopy (NIRS) is a relatively new tool that allows continuous noninvasive monitoring of in vivo oxygenation in selected tissues such as muscle and brain. Since hemoglobin, myoglobin, and cytochrome c oxidase are the only biological compounds to exhibit variable absorption of near-infrared (NIR) light in response to changes in oxygen availability, NIRS can determine changes in tissue oxygenation. NIRS can measure regional blood volume, local oxyhemoglobin and deoxyhemoglobin contents, and reduction-oxidation state of cellular mitochondrial cytochrome a, a3. As a comprehensive monitor of regional oxygen metabolism, NIRS has been applied in certain clinical and research settings. Despite technical limitations and the lack of definite "gold standards" to allow validation of results, NIRS remains a promising technology with applications in both the critical care environment and the research laboratory studying mechanisms of oxygen metabolism.