The one guaranteed property of enzymes isolated from extremely thermophilic micro-organisms is their thermostability. Most significantly, almost any such enzyme will be more thermostable than the functionally similar enzyme from a lower temperature source. Thermostability is not an isolated property: resistance to heat denaturation imparts stability to a number of other denaturing influences (detergents, organic solvents, etc). These characteristics of hyperthermophilic enzymes are the most likely basis for the development of new biotechnological applications. A limited number of hyperthermophilic enzymes have found application in specialist biotechnological applications; others have visible potential in growing areas of biotechnology. Existing and potential applications are discussed using DNA manipulation enzymes, dehydrogenases, and esterases as examples.