This book intends to provide an overview of current research on halophilic microorganisms, highlighting the diversity of life forms adapted to tolerate high salt concentrations and low water activities. Most of the 35 chapters are based on lectures presented during the international symposium 'Halophiles 2004', held in Ljubljana, Slovenia, in September 2004. Descriptions of the diverse high-salt environments in which halophiles are found are followed by sections devoted to the properties of halophilic Archaea, of halophilic and halotolerant Bacteria, and of different groups of salt-loving Eukarya, including fungi, algae and protozoa. Extensive information is provided about fungi adapted to life at high salt concentrations, a group that was poorly known until very recently. This volume is intended for researchers and students interested in a wide range of disciplines in the life sciences: from microbial ecology and adaptation of microorganisms to life in extreme environments to genomics, biotechnology, and astrobiology.