The pulse oximeter is an instrument that incorporates both oximetry and plethysmography to provide continuous non-invasive monitoring of oxygen saturation in patients who require such monitoring. Oxygen saturation values obtained from pulse oximetry are part of a complete assessment of a patient’s oxygenation status but are not a substitute for full arterial blood gas analysis. Pulse oximetry can be useful in early detection of arterial hypoxemia in many areas of clinical practice; however certain limitations from either technical or patient-related sources must also be considered. The principle, limitations and clinical applications of pulse oximetry are reviewed in this article.