This volume is concerned with providing a modern account of the theory of rates of diffusion-controlled reactions in solution. A brief elementary discussion of this area appeared in Volume 2 of this series, which was published in 1969. Since then, the subject has undergone substantial development to the point where we consider it timely that a complete volume devoted to the field appears. Unlike previous volumes of Comprehensive Chemical Kinetics, Volume 25 has been written entirely by one author, Dr. Rice, and his view of the objectives and scope of the book are summarised in Chapter 1.