This thesis presents and summarizes current knowledge on fundamental adaptations of organisms to life in extreme ecological conditions. In an anthropocentric view, environments hostile to man were designated as extreme. Some of them have originally been considered too extreme to support microbial life. However, organisms have evolved and live in such environments and they are called extremophiles. Depending on their optimal growth conditions, they are named thermophiles, psychrophiles, acidophiles, halophiles, etc. Adaptations in mentioned organisms are diverse and some of them are synthesis of enzymes with specific structural features, DNA repair mechanisms, unique cell membranes and regulation of body size. In this paper examples of experiments conducted on useful models to explore morphological and physiological adaptations are shown. Tardigrades are microscopic animals uniquely adapted to a range of environmental extremes. It is shown that they can survive extremes entering cryptobiosis but even in their active states they can be extremely tolerant. The Mediterranean shrub colonizes extreme geothermal fields. The analyses of its leaves showed stress-induced adaptations involved in their surviving. Multidisciplinary research on extremophiles allows understanding how life appeared and evolved on the only planet where it is so far known and it is crucial if we are to look for life in space.