Microorganisms from extreme environments have attracted the attention of many scientists and are currently one of the main focuses of research due to the fascinating mechanisms that they display in order to live under extreme conditions.One of the basic aspects of study in extremophiles is to unravel the adaptation mechanisms of these microorganisms to the severe conditions of temperature,pH or salinity of extreme habitats,and certainly the results will permit interesting developments in the future.Moreover,the extensive studies carried out during recent years have made it possible to use them as cell f- tories to produce new compounds of biotechnological importance and to develop new strategies for industrial processes. Besides thermophiles,hyperthermophiles,alkaliphiles,acidophiles or p- chrophiles,one of the most interesting groups of extremophiles is costituted by the halophilic microorganisms.These organisms require high levels of salt to grow and are common inhabitants of hypersaline environments, such as lakes,salterns or saline soils.This book is devoted to the salt-loving mic- organisms and focuses on different aspects of halophilic Archaea, Bacteria and Eukarya.