This book explains the basic principles underlying the use of supercritical fluids. It presents the relevant physical chemistry and the unusual behavior obtained in the medium. Although topics are discussed completely, the detail is kept to a minimum and the focus throughout remains on basic principles, supplemented by experimental examples. Some of the topics, including principles of supercritical fluid fractionation, are not covered elsewhere in the literature. The book begins with basic topics: phase behavior, dynamic properties, transport properties, and the effects of intermolecular forces, structures and fluctuations. The second half then develops principles widely used in the field, particularly separation methods and chemical reactions.