Pulsimetry is a non-invasive technique to monitor the saturation of oxygen in the hemoglobin of blood in arteries. Initially. This was used by anesthesiologists to control oxygen levels in patients undergoing general anesthesia and in the care of critically ill patients. Given its effectiveness to detect precociously situations of hypoxia, its use has rapidly expanded during the decade of the '90s to other units having patients who need a correct measure of their respiratory functions. As with any technique, it is necessary to understand the optimal conditions for its application, its technical principles and the limitations this instrument has, as well ass those internal or external circumstances which may interfere in the values offered by the pulsimeter in order to avoid errors while interpreting these measurements.