Extremes of temperature are likely to increase in frequency associated with climate change. Older patients are particularly vulnerable to the effects of heat with excess mortality well documented in this population. Age-associated neurohormonal changes particularly affecting the renin angiotensin aldosterone system (RAAS), alterations in thermoregulatory mechanisms, changes in renal function and body composition render older persons vulnerable to dehydration, renal failure, heat stroke and increased mortality. Barriers to diagnosis and recognition of dehydration and renal failure include the absence of reliable clinical signs and cost-effective diagnostic tools. Regularly used medications also impact on physiological responses to excess heat as well as interfering with the recognition and management of dehydration during heat waves. In view of the above, anticipatory measures should be instituted ideally prior to the onset of heat waves to minimise morbidity and mortality for older people during periods of excess heat.