Welcome to the World Health Organization pulse oximeter training manual. In many countries pulse oximetry is mandatory for monitoring patients during anaesthesia. Although pulse oximetry is a simple and reliable technology that can detect low levels of oxygen in the blood, it is only effective if the anaesthesia provider understands how an oximeter works and what to do when hypoxia is detected. This manual describes a simple plan to respond to this situation, and explains how oximeters work and how to use them. The manual contains essential information for all anaesthesia providers who are not experienced in using pulse oximetry and would be useful reading for all members of the theatre team.