The Archaea are a group of single-celled micro-organisms. They have no cell nucleus or any other organelles within their cells. In the past they were viewed as an unusual group of bacteria and named archaebacteria but since the Archaea have an independent evolutionary history and show many differences in their biochemistry from other forms of life, they are now classified as a separate domain in the three-domain system. Archaea include micro-organisms adapted to a variety of environmental conditions, including some of the most hostile locations on Earth. This book presents current research in the study of Archaea from across the globe, including the Archaeal flagellum; the Archaea ribonuclease P; mechanosensitive channels in Archaea; and the physiology, genomics and phylogeny of nanoarchaeota.