Heat exhaustion and heatstroke are part of a continuum of heat-related illness. Both are common and preventable conditions affecting diverse patients. Recent research has identified a cascade of inflammatory pathologic events that begins with mild heat exhaustion and, if uninterrupted, can lead eventually to multiorgan failure and death. Heat exhaustion is characterized by nonspecific symptoms such as malaise, headache, and nausea. Treatment involves monitoring the patient in a cool, shady environment and ensuring adequate hydration. Untreated heat exhaustion can progress to heatstroke, a much more serious illness involving central nervous system dysfunction such as delirium and coma. Other systemic effects, including rhabdomyolysis, hepatic failure, arrhythmias, disseminated intravascular coagulation, and even death, are not uncommon. Prompt recognition and immediate cooling through evaporation or full-body ice-water immersion are crucial. Physicians also must monitor electrolyte abnormalities, be alert to signs of renal or hepatic failure, and replace fluids in patients with heatstroke. Most experts believe that physicians and public health officials should focus greater attention on prevention. Programs involving identification of vulnerable individuals, dissemination of information about dangerous heat waves, and use of heat shelters may help prevent heat-related illness. These preventive measures, when paired with astute recognition of the early signs of heat-related illness, can allow physicians in the ambulatory setting to avert much of the morbidity and mortality associated with heat exhaustion and heatstroke.