Oxygen is essential for the processes of cellular respiration and for life. Pulse oximeters enable routine monitoring of patient oxygenation during and after anesthesia and in many other applications in medicine. Their development and use in clinical practice has represented a major advance in patient safety. The ability to deliver oxygen and measure its delivery is essential to the well-being of patients. This is especially true in anesthesiology and perioperative care, where patients are at risk of hypoxemia and other ventilatory impairments. Pulse oximetry represents a major advance in patient safety whose importance cannot be over emphasised. Many new and emerging oximetry technologies are being developed which may further benefit our care of patients. A proper understanding of the use and interpretation of pulse oximetry, as well as its utility in many aspects of patient care, is essential to our practice of medicine. A detailed discussion on co-oximetry, and other newer technologies has been given in this chapter.