Book excerpt: Conjugated polymers have important technological applications, including solar cells and light emitting devices. They are also active components in many important biological processes. In recent years there have been significant advances in our understanding of these systems, owing to both improved experimental measurements and the development of advanced computational techniques. The aim of this book is to describe and explain the electronic and optical properties of conjugated polymers. It focuses on the three key roles of electron-electron interactions, electron-nuclear coupling, and disorder in determining the character of the electronic states, and it relates these properties to experimental observations in real systems. A number of important optical and electronic processes in conjugated polymers are also described. The second edition has a more extended discussion of excitons in conjugated polymers. There is also a new chapter on the static and dynamical localization of excitons.