Most orgnisms and populations have to cope with hostile environments, which may threaten their existence. Their ability to respond phenotypically and genetically to these challenges and to evolve adaptive mechanisms is therefore crucial. The aim of this text is the understanding of the impact of stress on biological systems, from an evolutionary perspective. It discusses how organisms adapt to extreme environments, how stress affects life histories and changes genetic structure, how organisms cope with thermal stress through acclimatization, and how environmental and genetic stress induces fluctuating asymmetry, shapes selection pressures and causes extinction of populations. These topics are discussed through a variety of approaches, from the molecular and the protein level to individuals, populations, and ecosystems. The text also considers the role of stress in evolutionary change, from stress-induced mutations to selection, speciation and evolution on the geological time scale.