Heat-related illnesses comprise a spectrum of syndromes resulting from disruption of thermoregulation in people exposed to high environmental heat. Symptoms range from heat edema and exercise-associated muscle cramps to exercise-associated collapse, heat exhaustion, and life-threatening heat stroke. Athletes, outdoor laborers, and military personnel are at greatest risk. Several intrinsic and extrinsic factors increase the risk of heat-related illness, including medical conditions, environmental factors, medication use, and inadequate acclimatization. Proper recognition and treatment are effective in preventing adverse outcomes. Management of the mildest forms of heat-related illness (e.g., heat edema, exercise-associated muscle cramps) is largely supportive, and sequelae are rare. Heat exhaustion is characterized by cardiovascular hypoperfusion and a rectal core temperature up to 104°F without central nervous dysfunction. Mild cooling, rest, and hydration are recommended. Heat stroke is a medical emergency in which patients present with rectal core temperature of 105°F or greater, multiorgan damage, and central nervous dysfunction. Ice water or cold water immersion is recommended. Patients adequately cooled within 30 minutes have excellent outcomes. Patients with heat stroke generally require hospitalization to monitor for medical complications despite rapid cooling. People diagnosed with heat stroke or severe heat-related illness should refrain from physical activity for at least seven days after release from medical care, then gradually begin activity over two to four weeks. Acclimatization, adequate hydration, and avoidance of activities during extreme heat are the most effective measures to reduce the incidence of heat-related illnesses.