The pulse oximeter has been shown to be a reliable monitor of arterial oxygen saturation and has therefore been recommended as mandatory monitoring for patients during anaesthesia and intensive care. In 1989 two review articles on pulse oximetry were published (1, 2) and two years ago Severinghaus and Kelleher summarized the literature between 1989 and October 1991 (3). Our aim is to focus the discussion on technical aspects and applications of pulse oximetry with special attention centered on recent developments. This review is consequently an update on pulse oximetry since the end of 1991, and the first on technically-based publications in the two last decades.