This article reviews the application of near infrared (NIR) spectroscopy in preterm and newborn infants and its clinical value. After an overview of the commercially available instrumentation, which is able to provide absolute values of tissue haemoglobin saturation (StO(2)), we focus on the application of NIR spectroscopy to measure brain, liver, gastro-intestinal and peripheral oxygenation. From the instrumentation point of view, NIR spectroscopy has gained significantly in clinical importance, especially by being able to measure 5tO(2), i.e. an absolute parameter. An increasing number of commercial instruments is available today and the number of users is virtually exploding. However, the precision of the measurements is still too low from a clinical point of view and the variety of algorithms employed by the different instruments and sensors may provide quite different StO(2) values. In the future, the different instruments need to report quantitatively comparable StO(2) measurements which are more precise. The measurement of cerebral StO(2) may be useful for detecting situations where the oxygenation of the brain may be impaired and the hope is that this indicates situations which lead to brain lesions. However, the clinical utility of cerebral StO(2) still needs to be examined. Although liver and gastro-intestinal as well as peripheral measurements are still an object of research and several problems have to be overcome before clinical use, these measurements could be of high value in specific clinical situations.