The cryosphere stands for environments where water appears in a frozen form. It includes permafrost, glaciers, ice sheets, and sea ice and is currently more affected by Global Change than most other regions of the Earth. In the cryosphere, limited water availability and subzero temperatures cause extreme conditions for all kind of life which microorganisms can cope with extremely well. The cryosphere's microbiota displays an unexpectedly large genetic potential, and taxonomic as well as functional diversity which, however, we still only begin to map. Also, microbial communities influence reaction patterns of the cryosphere towards Global Change. Altered patterns of seasonal temperature fluctuations and precipitation are expected in the Arctic and will affect the microbial turnover of soil organic matter (SOM). Activation of nutrients by thawing and increased active layer thickness as well as erosion renders nutrient stocks accessible to microbial activities. Also, glacier melt and retreat stimulate microbial life in turn influencing albedo and surface temperatures. In this context, the functional resilience of microbial communities in the cryosphere is of major interest. Particularly important is the ability of microorganisms and microbial communities to respond to changes in their surroundings by intracellular regulation and population shifts within functional niches, respectively. Research on microbial life exposed to permanent freeze or seasonal freeze-thaw cycles has led to astonishing findings about microbial versatility, adaptation, and diversity. Microorganisms thrive in cold habitats and new sequencing techniques have produced large amounts of genomic, metagenomic, and metatranscriptomic data that allow insights into the fascinating microbial ecology and physiology at low and subzero temperatures. Moreover, some of the frozen ecosystems such as permafrost constitute major global carbon and nitrogen storages, but can also act as sources of the greenhouse gases methane and nitrous oxide. In this book we summarize state of the art knowledge on whether environmental changes are met by a flexible microbial community retaining its function, or if the altered conditions also render the community in a state of altered properties that affect the Earth's element cycles and climate. This book brings together research on the cryosphere's microbiota including permafrost, glaciers, and sea ice in Arctic and Antarctic regions. Different spatial scales and levels of complexity are considered, spanning from ecosystem level to pure culture studies of model microbes in the laboratory. It aims to attract a wide range of parties with interest in the effect of climate change and/or low temperatures on microbial nutrient cycling and physiology.