Pulse oximetry allows a rapid noninvasive estimate of arterial oxygen saturation. Since its development in the 1970s, it has made a significant impact, particularly in the fields of perioperative and intensive care medicine. Most modern pulse oximeters determine arterial hemoglobin saturation through the use of two light-emitting diodes in the red (660 nm) and infrared (940 nm) spectrum. The differential absorption of these two wavelengths of light by oxygenated and deoxygenated hemoglobin during pulsatile blood flow allows for accurate estimation of arterial oxygen saturation under most conditions. However, pulse oximeters can give erroneous from a variety of causes, including hypoperfusion, dyshemoglobins (including carboxyhemoglobin), nail polish, darker skin pigmentation, venous pulsations, and, perhaps most frequently, motion artifact. Clinicians must consider these possible causes of error when interpreting pulse oximetry results, especially those that are not consistent with a patient’s clinical status and medical history. Clinically significant desaturations in an ambulatory setting are uncommon in patients without significant pathologic pulmonary conditions or pulmonary vascular disease.