In 2018, approximately 2.8 million passengers flew in and out of U.S. airports per day. Twenty-four to 130 in-flight medical emergencies are estimated to occur per 1 million passengers; however, there is no internationally agreed-upon recording or classification system. Up to 70% of in-flight emergencies are managed by the cabin crew without additional assistance. If a health care volunteer is requested, medical professionals should consider if they are in an appropriate condition to render aid, and then identify themselves to cabin crew, perform a history and physical examination, and inform the cabin crew of clinical impressions and recommendations. An aircraft in flight is a physically constrained and resource-limited environment. When needed, an emergency medical kit and automated external defibrillator are available on all U.S. aircraft with at least one flight attendant and a capacity for 30 or more passengers. Coordinated communication with the pilot, any available ground-based medical resources, and flight dispatch is needed if aircraft diversion is recommended. In the United States, medical volunteers are generally protected by the Aviation Medical Assistance Act of 1998. There is no equivalent law governing international travel, and legal jurisdiction depends on the patient's and medical professional's countries of citizenship and the country in which the aircraft is registered.