Category 1 Continuing Medical Education credit for physicians is available to Wilderness Medical Society members for this article. Go to http//wms.org/cme/cme.asp?whatarticle=1843 to access the instructions and test questions.Objective.—Expedition activities such as mountaineering, rock climbing, river running, sea kayaking, and canoeing all involve an element of risk. Organizations that provide group wilderness and adventure experiences are responsible for managing the risk of their courses. The leaders and medical providers of these trips must therefore be prepared to anticipate and manage medical problems that may arise. The aim of this study is to provide the medical community with a better understanding of the specific injuries and illnesses that occur on wilderness expeditions.Methods.—A retrospective descriptive study was done examining the medical incidents that occurred on wilderness-based courses during the 3-year period from September 1, 2002, through August 31, 2005. Participants and staff of the National Outdoor Leadership School (NOLS) served as the study population.Results.—Injuries occurred at a rate of 1.18 per 1000 program days, and illnesses at a rate of 1.08 per 1000 program days. There were no fatalities during the time period. Athletic injuries (sprains, strains) and gastrointestinal illnesses were the most common medical incidents. Hypothermia, seizures, appendicitis, heat stroke, and pregnancy occurred but with low frequency. Fractures, dental emergencies, tick fever, athletic injuries, and nonspecific body pains were the conditions most frequently requiring evacuation.Conclusions.—The rate of medical incidents on NOLS courses declined during the 1990s and has remained relatively steady apart from a slight increase in 2004 and 2005. Athletic injuries continue to be a difficulty, as they frequently result in evacuation even though their ultimate outcome is usually benign. Evacuation decisions should be made considering both the potential severity of the medical condition as well as patient comfort. Wilderness medical personnel must be familiar with a diverse range of medical conditions in order to provide optimal care.