The purpose of this book is to explain the basic physical principles underlying the use of supercritical fluids. The relevant areas of physical chemistry are adapted to the context of supercritical fluids and attention is drawn to the special behaviour obtained in the medium. Although the areas are discussed comprehensively, description of all the complexities possible is avoided, so that the basic principles are clear. Experimental examples are used only to illustrate the principles and there is no attempt to review all the work published. Some of the topics, and in particular the principles of supercritical fluid fractionation, are not presently covered in other books or published in the refereed literature. The first half of the book covers the very basic topics of phase behaviour, thermodynamic properties, transport properties, especially diffusion, the effect of intermolecular forces, structure and fluctuations. In the second half, some topics in which these principles have been widely applied in the literature are described, namely separation methods and chemical reactions.