This volume is an interdisciplinary treatise on the theoretical approach to solvation problems. Solvents are important in chemistry, physics, and biology. Almost all experiments made in organic chemistry, including all kinds of spectroscopy, are made in solution. In biology, several reactions are known that only occur in an aqueous environment. In physics, the interaction with the environment changes optical properties and affects the design of sensors and devices. In recent years the development of theoretical methods to treat solvation problems has seen great progress and quantum chemistry is now making an important incursion into the liquid phase, coupling statistical physics with quantum mechanics. This book describes the essential details of the theoretical methods and places them into the context of modern applications, and hence is of broad interest to theoreticians and experimentalists in both academic and industrial settings. The assembly of these modern methods and applications into one volume is a unique contribution to date, and is likely to have large impact on the community that is making rapid advances in science and engineering. All of the proposed authors are renowned experts in their fields and the different topics covered in this volume represent complimentary approaches that will give a broad and ample description of the field in its present stage of development.