The pulse oximeter has revolutionized modern medicine with its ability to continuously and transcutaneously monitor the functional oxygen saturation of hemoglobin in arterial blood (SaO2). Studies have shown that the presence of pulse oximetry may reduce the number of arterial blood gas samples obtained in the ICU and in the emergency department 2). Pulse oximetry is universally used for monitoring respiratory status of patients in the ICU (intensive care unit). Pulse oximeter is a noninvasive method for monitoring a person’s oxygen saturation in arterial blood (SaO2) that is frequently used in hospital but are rarely used at home 1). The science behind Oxyhemoglobin Dissociation curve.Pulse oximetry effect on clinical outcome.Causes of falsely low or high pulse oximeter SpO2.Causes of falsely low pulse oximeter readings.Causes of falsely normal or elevated SpO2 pulse oximetry value.Causes of intermittent drop-outs or inability to read pulse oximeter SpO2.Pros and cons of different types of pulse oximeter probes.