In this chapter the current knowledge of the diversity of piezophiles isolated so far is reviewed. The isolated piezophiles originated from high-pressure environments such as the cold deep sea, hydrothermal vents, and crustal rocks. Several "stress" conditions can be experienced in these environments, in particular high hydrostatic pressure (HHP). Discoveries of abundant life in diverse high-pressure environments (deep biosphere) support the existence and an adaptation of life to HHP. At least 50 piezophilic and piezotolerant Bacteria and Archaea have been isolated from different deep-sea environments but these do not by far cover the large metabolic diversity of known microorganisms thriving in deep biospheres. The field of biology of piezophiles has suffered essentially from the requirements for high-pressure retaining sample containers and culturing laboratory equipment, which is technically complicated and expensive. Only a few prototypes of HHP bioreactors have been developed by a number of research groups and this could explain the limited number of piezophiles isolated up till now.