The group of extremophilic bacteria includes all microorganisms which are usually isolated from habitats generally retained as extreme to the humans as well as to the majority of higher organisms. These peculiar ecosystems can be described according to physical and chemical parameters: elevated temperature (hydrothermal vents, sulfataric waters, geysers), NaCl concentration > 10-15% (salterns, deserts), hydrostatic pressure (ocean dephts), low temperature (aquatic environments, frozen areas, artic zones), anoxic environments (superficial and aquatic mud layers, municipal waste digesters, gastro-intestinal system of higher and lower organisms). These microorganisms are therefore very interesting for taxonomy as well as for ecology. During the last 10-15 years, many species of extremophiles were described and considered for potential applications due to their unique physiological and biochemical features. In this short review, our aim is to describe, besides the many ecosystems involved, some species of extremophilic microorganisms which are particularly interesting for basic research and for potential applications as well.