Climate change, characterized by increased mean temperatures and more frequent occurrence of extreme temperatures, may lead to adverse health effects through different and complex mechanisms due to substantial changes in the physical and social environment. The most easily measurable health effects of climate change in high-risk groups are the direct effects of excessively high temperatures. A literature review was performed to identify studies regarding heat waves, risk factors and prevention programmes. A large number of studies performed in Europe during the August 2003 heat wave, and previously in both Europe and North America, showed excess mortality during periods of extreme heat. The role of pollution as a confounding factor remains dubious. Groups at risk include the elderly, the urban population, individuals with impaired health and those with low income. Aging of the population (especially in industrialised nations) and increased urbanization (especially in developing countries) may further increase ill health effects of high temperatures.