Conjugated polymers have important technological applications, including solar cells and light emitting displays. They are also active components in many 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 character and energetic ordering of the electronic states and relates these properties to experimental observations in real systems.