The causes of hypoxaemia in children include the commonest causes of childhood illness: pneumonia and other acute respiratory infections, and neonatal illness, particularly sepsis, low birthweight, birth asphyxia and aspiration syndromes. The systematic use of pulse oximetry to monitor and treat children in resource-poor developing countries, when coupled with a reliable oxygen supply, improves quality of care and reduces mortality. Oximetry also has a well established role in surgery and anaesthesia, but in many countries children undergo surgery without the safety of oximetry monitoring. This article reviews pulse oximetry, its technical basis and its application to the medical management of childhood illness to reduce mortality in developing countries. We propose that, as a part of the work towards achieving the Millennium Development Goal 4, there should be a concerted global effort to make pulse oximetry and a reliable oxygen source available in all health facilities where seriously ill children are managed.