Oximetry is used to quantify the presence of oxygen in human blood within soft tissues of the human body. Among multiple implementations of this technology, pulsatile oxygen saturation (SpO2) is a core medical technology and is being rapidly adopted in consumer health. However, despite its long history of clinical use, recent findings indicate that the accuracy of pulse oximetry may be affected by various factors and biases. For example, the COVID-19 pandemic showed that pulse oximeters exhibited flaws in accuracy due to the skin pigmentation of patients with darker skin. Thus, the future of this technology, particularly in consumer health devices, needs to be built on foundations that account for such biases. This chapter reviews the principles of pulse oximetry, sources of its artifacts, calibration methods, and the factors that may cause inaccuracy in pulse oximeters, particularly pertinent to two-wavelength pulse oximetry. Drawing upon recent research and clinical insights, we review the multifaceted nature of pulse oximetry biases, including motion artifacts, skin pigmentation, body mass index, environmental variables, device calibration, and nail polish, among others.