Microorganisms perform many important functions in Arctic and Antarctic environments, but their activity and occurrence can be difficult to detect. At sub-zero temperatures, many bacteria retain their viability, but they may stay inactive for long periods. In this review, we describe essential elements of adaptation, abundance and activity of microorganisms in Arctic and Antarctic environments, and we give examples on their participation in key biogeochemical processes in permafrost soils, snow and ice. Microbes have adapted to low temperatures by adjusting the content of fatty acids and proteins in the cell membranes, and some bacteria produce a thicker cell wall. In the cytoplasm, cryoprotectants reduce freezing effects, chaperones ensure correct folding of macromolecules, and enzyme are optimized by lowering enthalpy and increasing the proportion of α- vs β-helices. Abundant microbial taxa in cold environments often belong to Proteobacteria, Actinobacteria and Acidobacteria, possibly due to their ability to survive at low nutrient levels and a high metabolic diversity, e.g., production of extracellular enzymes and fixation of N₂. Some bacteria demonstrate growth at subzero temperatures (lower than − 10 °C) by adjusting vital processes, e.g., protein synthesis, maintenance metabolism and by reducing mobility. Important microbial ecosystem functions are exemplified by cycling of methane in oxic and anoxic conditions and by bioremediation of oil spills by indigenous microbes. Further, the significance of microbial transformation of the pollutants mercury, polychlorinated biphenyls (PCBs) and perfluorinated alkylated substances (PFAS) in polar regions is discussed.