Life on our planet has adapted to a wide range of physical conditions, including extremely high and low temperatures, high pressures, extremes of pH and chemically aggressive conditions. To cope with these stress factors, organisms have evolved a variety of strategies operating on very different levels, from the small molecule response to physiological and behavioural adaptation. The only kind of stress response that is found universally in all species is the stress-induced expression or overexpression of specific proteins. Among these, the heat shock proteins are the best studied group. They have been shown to serve in a variety of specific functions, including those of molecular chaperones, proteases, and "capacitors of evolution." An overview of these different functions and also of the other kinds of stress proteins is given, with a perspective on how they serve the survival of the cell and the species in the presence of environmental stress factors, and how they can be used in medical applications.