Although they comprise one of the three fundamental branches of life, the Archaea were only recognized as a group about twenty years ago. This recognition was based on similarities between their RNA sequences, similarities all the more striking because of the diversity of archaeal lifestyles. They include microorganisms that live in boiling water, within the guts of animals, or in concentration brines. It is also evident that the Archaea diverged early in the history of life, and are of great evolutionary interest as a result. This book tells their emerging story, from their toughness in the face of forbidding environments to their unique place in evolution and in the world ecosystem.