Blood oxygen saturation indicates the percentage of hemoglobin molecules saturated with oxygen. It is a vital sign of a patient's health, considering that without a constant oxygen supply, a human being cannot survive for a prolonged time. Blood oxygen saturation measurements can be categorized into chemical methods and optical methods. Chemical methods need a blood sample for in vitro analysis, whereas optical methods measure blood oxygen saturation by passing specific wavelengths of light through the blood. As optical methods can achieve measurements noninvasively, they have been widely used in clinical applications. Among the optical methods, pulse oximetry, which can measure blood oxygen saturation with comparable accuracy, is the one that has been widely used in clinical environments. Beyond its traditional applications for adults on extremities, pulse oximeters for fetal use and measurement at the retina are under active development.