It is now common for major athletic contests to take place in conditions of high heat and humidity. This poses a major challenge for competitors, as the performance of both physical and mental tasks can be adversely affected by heat and by dehydration. The increased sensation of effort during exercise in the heat is also a challenge to the promotion of health-related physical activity in hot climates. There are well-recognised effects of heat and hydration status on the cardiovascular and endocrine systems, and these effects play a role in the decreased performance experienced in the heat. However, growing evidence from various experimental models suggests that the primary effects of high ambient temperature and dehydration on performance may be mediated, at least in part, by effects on the central nervous system. This seems to be a consequence of changes in serotonergic and dopaminergic function, with pharmacological manipulation of these neurotransmitters demonstrated to influence exercise performance. Recent evidence suggests that the integrity of the blood brain barrier may be compromised by combined heat stress and dehydration, and this may also play a role in limiting performance in the heat. Coping strategies include increased aerobic fitness, prior exposure to exercise/heat stress resulting in acclimatisation, pre-exercise cooling, and hydration. Provision of fluids of an appropriate composition can prevent dehydration and can greatly reduce the adverse effects of heat stress.