Extremophiles is a component of Encyclopedia of Biological, Physiological and Health Sciences in the global Encyclopedia of Life Support Systems (EOLSS), which is an integrated compendium of twenty one Encyclopedias. The extremophiles represent some of the most fascinating organisms on Earth for the simple reason that they inhabit extreme environments characterized by physical and (or) chemical properties which render them totally inhospitable for most of the other organisms. The work has been sub-divided into 6 main topics related to the above mentioned environmental conditions. These topics consist of a general introduction and of several more specialized chapters that have been written by scientists prominent in the field. The chapters cover the description of the biotopes and inhabiting species, their specific characteristics as well as what we know about the molecular mechanisms which constitute the fundamentals of the resistance and adaptation of extremophiles to extreme conditions. The theme “Extremophiles” is headed by two chapters introducing the subject for non-specialists in the field, one covering the basic concepts and the other one giving an overview of the biotopes. These three volumes are aimed at the following five major target audiences: University and College students Educators, Professional practitioners, Research personnel and Policy analysts, managers, and decision makers and NGOs.