Circulating oxygenated blood to different body parts and organs is vital for the energy generation and survival of the human body. Blood pumped by heart circulates through the fine capillaries in lungs where the red blood cellsRed blood cells absorb oxygen from the air inhaled. In a healthy person, it is expected that 95–100% red blood cells get saturated with oxygen when passing through the lungs. However, some ailments reduce the oxygen saturation level of blood, and it is therefore required to monitor the oxygen saturation level of blood frequently in such cases. Since the oxygen intake by red blood cellsRed blood cells also depends on the breathing rate, it is required to monitor the breathing rate also. Pulse oximeter serves the dual purpose of monitoring the oxygen saturation and the breathing rate in a patient. The instrument works on the principle that the oxygenated and the unsaturated red blood cells have different values for the absorption of the red-light and the infrared light. Details of the method along with the constituents of the human blood and its grouping are discussed in the chapter.