Extremophiles are organisms which inhabit environments characterized by properties harsh enough to hinder the survival of common cells. They are highly diversified and are classified on the basis of the main extreme property that prevails in the habitat. Six main categories can be distinguished: the thermophiles found in high temperature sites and which can tolerate temperatures sometimes close to that of the boiling point of water; the psychrophiles living in permanently cold habitats with temperatures sometimes well below the freezing point of water; the piezophiles, which tolerate pressure as high as 1000 atm; the halophiles supporting salt concentrations, in some cases, higher than 300 g l−1; the acidophiles thriving well at pH sometimes close to zero; and the alkaliphiles, which, on the contrary, tolerate pH largely exceeding neutrality. These organisms are mainly microorganisms and they notably produce enzymes that are adapted to work in unusual conditions often required in biotechnological processes. This confers upon these organisms a very high potential. They are the target of a steadily increasing interest and are nowadays largely used in various industrial applications.