Pulse oximetry has become the leading technique for monitoring hemoglobin oxygen saturation whenever a risk of hypoxia exists. It is based upon the principle of light absorbance of pulsatile arterial blood. Since it uses 2 wavelengths, it can only measure oxyhemoglobin and reduced hemoglobin. Thus, in case of increased methemoglobin or carboxyhemoglobin, pulse oximetry values are not reliable. Low tissue perfusion can also be responsible for inaccurate values. Nevertheless pulse oximetry is a reliable and easy-to-use technique in most of the clinical situations. This article reviews the physical basis, the reliability, the pediatric aspects, and the limitations of the technique.