Pulse oximetry is often perceived to be a measure of the adequacy of oxygen delivery. It is, however, only a measure of oxygen bound to haemoglobin. Systemic oxygen delivery is principally determined by cardiac output, haemoglobin concentration and haemoglobin saturation. Changes to both cardiac output and haemoglobin concentration will significantly alter oxygen delivery without changing oxygen saturation. This article will describe the components of systemic oxygen delivery and the physiologic limitation of pulse oximetry and caution against over-interpretation of oximetry in the care of newborns.