Tests relying on the passage of the light through a tooth have been considered to be a suitable means of assessing pulp vitality in children. Pulse oximetry which is an effective, objective oxygen saturation monitoring technique broadly used in medicine for recording blood oxygen saturation levels, can also be used in endodontic diagnosis for differential diagnosis of vital and necrotic pulps in young teeth. In this system, light is passed from a photoelectric diode across the tooth structure into a receptor. The instrument detects changes in absorption in both red and infrared light caused by alteration in tissue volume during the cardiac cycle. However, there are some limitations such as the effect of increased acidity and metabolic rate which cause deoxygenating of hemoglobin and changes in the blood oxygen saturation. Because this test produces no noxious stimuli, children usually accept it more readily than routine methods. In this lecture, principles, indications, limitations, influencing factors, and variations in probe design for dental usage in children would be presented.