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Cryogenic Avalanche Detectors (CRADs) are referred to as a new class of noblegas detectors operated at cryogenic temperatures with electron avalanching performed directly in the detection medium, the latter being in gaseous, liquid or two-phase (liquid-gas) state. Electron avalanching is provided by Micro-Pattern Gas Detector (MPGD) multipliers, in particular GEMs and THGEMs, operated at cryogenic temperatures in dense noble gases. The final goal for this kind of detectors is the development of large-volume detectors of ultimate sensitivity for rare-event experiments and medical applications, such as coherent neutrinonucleus scattering, direct dark matter search, astrophysical (solar and supernova) neutrino detection experiments and Positron Emission Tomography technique. This review is the first attempt to summarize the results on CRAD performances obtained by different groups. A brief overview of the available CRAD concepts is also given and the most remarkable CRAD physics effects are discussed.Earlier attempts to obtain high and stable electron avalanching directly in noble gases and liquids at cryogenic temperatures, using "open-geometry" gaseous multipliers, have not been very successful: rather low gains (≤10) were observed in liquid Xe [2],[3],[4] and Ar [2],[5] and low gains (≤100) in gaseous Ar [6] and He [7] at low temperatures, using wire, needle or microstrip proportional counters. Moreover, two-phase detectors with wire chamber readout, which initially seemed to solve the problem, turned out to have unstable operation in the avalanche mode due to vapour condensation on wire electrodes [8].The problem of electron avalanching in cryogenic noble-gas detectors has been solved [1] after introduction of Micro-Pattern Gas Detectors (MPGDs), namely those of hole-type: Gas Electron Multipliers (GEMs) [9] and thick GEMs (THGEMs) [10]. Contrary to wire chambers and other "open geometry" gaseous multipliers, cascaded GEM and THGEM structures have a unique ability to operate in dense noble gases at high gains [11], including at cryogenic temperatures and in the two-phase mode [12].Consequently at present, the basic idea of Cryogenic Avalanche Detectors in the narrow sense is that of the combination of MPGDs with cryogenic noble-gas detectors, operated in a gaseous, liquid or two-phase mode. We call such detectors "CRyogenic Avalanche Detectors" and suggest the following short-name for those -CRADs; it will be used throughout in the following. This review is the first attempt to summarize the results on CRAD performances obtained by different groups, presenting those in a systematic way. A brief overview of the available CRAD concepts is also given and the most remarkable CRAD physics effects are discussed. Cryogenic Avalanche Detector (CRAD) concepts: brief overviewThere are at least a dozen of different CRAD types developed by different groups over the past 8 years. In this chapter this variety is systemized: a brief overview of CRAD concepts is given, namely that of the basic CRAD concepts and CRAD concepts re...