Electron spin relaxation has established itself as an important experimental method for studying the details of molecular motion in liquids, and as a harsh testing ground for theoreticians. The theo­ retical difficulties are connected with the complexity of the mole­ cular motion, and the theoretical interest lies not only in its im­ portant consequences for the interpretation of experiments, but also in the fascination of a system in which a well-defined quantum me­ chanical component is in interaction with a complex quasi-classical environment. It is because the theories are concerned with such dissimilar but connected systems that the techniques involved are so numerous. Many of the standard manipulations of quantum mechanics must be brought to bear, and at the same time they must be combined with statistical techniques which are often of considerable sophis­ tication. The purpose of this volume is to present a survey of these techniques and their application to spin relaxation problems. No single volume can be exhaustive, but we believe that the contri­ butions to this volume are sufficiently broad to show how those who are concerned with spin relaxation problems think about the subject and circumvent, or expose, its difficulties. The first few Chapters (I-V) review the basic quantum mechani­ cal and statistical manipulations which are often used.