Guidelines for preventing heat injury (HI) among military personnel are not directly applicable to civilian personnel. Military guidelines call for relatively large volumes of prophylactic water consumption and physical activity limitations depending on the wet bulb globe temperature. However, in civilian populations, there is an increased prevalence of HI risk factors: older age, medication use, especially anticholinergic and psychotropic medications, obesity, previous HI, and skin disorders. Although dehydration is a major contributor to HI in military situations, it is unlikely in classical heat stroke among civilians. Civilian guidelines are based on the heat index. Activity levels must be restricted more for civilians, and prophylactic water consumption (beyond replacing loss from sweat) is not necessary. This review discusses the pathophysiology of heat injury, contrasts the military and civilian approach to prevention of HI, and describes appropriate field intervention for HI.