Book excerpt: As part of its ongoing commitment to the nation's space program, NASA's medical leadership asked the Institute of Medicine (IOM) to review specific aspects of the scientific basis, policies, and procedures associated with the Longitudinal Study of Astronaut Health (LSAH). NASA created the LSAH in 1992 to address a variety of issues, including both the health of astronauts during space flight and the longer-term health issues that might be associated with space flight and flight training.