Cells have developed complex ways to respond to various stresses. Interestingly, stresses such as heat, ischaemia and radiation can induce different cellular responses depending on their strength. While a mild stress induces a protective heat shock response, a more potent stress stimulus induces apoptosis and an even stronger one leads to necrosis. The heat shock or stress response, ie the synthesis of heat shock proteins (Hsps, stress proteins) in response to a mild stress, allows cells to adapt to gradual changes in their environment and to survive in otherwise lethal conditions. The ability of Hsps to protect cultured cells from both apoptosis and necrosis has been well demonstrated. Novel data suggest an important protective role for them also in vivo as they can protect heart and brain against ischaemia and lungs and liver against sepsis. Moreover, they can render tumours resistant to anticancer therapy. These and other cytoprotective effects of Hsps make them tempting targets for therapeutic interventions in several diseases.