Pulse oximetry is a ubiquitous monitor in anesthesia and critical care and is often considered the fifth vital sign. Under conditions of normal perfusion and temperature, the finger probe is the most common and effective sensor. In the presence of hypotension, hypoperfusion,and hypothermia, however, the finger sensor is often unable to detect a pulsatile signal. Another site and sensor are necessary to monitor these patients effectively. This article describes the search for this site, the choice of the forehead, and preliminary data regarding the use of this sensor site.