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...