Publications concerning in-flight medical events that result in in-flight treatment and diversions have appeared in the lay and scientific literature. Several have been based on one event, or on data from one or two airlines, and consequently may draw conclusions that are not necessarily applicable throughout the industry. This guidance document has been developed primarily for medical practitioners who volunteer to provide assistance on board and for those who wish to understand the background to airline provision of on-board first aid and medical care. This document considers: what are "in-flight medical events" and how often they occur; on-board medical supplies; cabin crew training; automated external defibrillators, and legal aspects. A checklist is provided for medical professionals called to provide assistance during an in-flight event.