This book begins with a short recapitulation of the equation of state for a real gas and a perfect gas. This is followed by the description of a liquid, an outline of the difficulties that would arise from a gas-like description and the reason why a solid-like description would seem more appropriate. An introduction to different cell and hole models for liquids, polymeric liquids and their mixtures is also included. The theories are developed from the basics, after which the book delves into more complex theories. These features of the book will appeal to both students and scientists who want to apply the equation of state theories. A wide range of applications to polymeric -- and also some non-polymeric -- systems is given. Finally, a thorough comparison of some of the theories is made.