Since the discovery of the light emitting properties of the phenyl-based organic semiconductors in 1990 there has been a huge growth of interest in conjugated polymers. The potential device applications are enormous, ranging from optical switching to solar cells and light emitting devices. They are also active components in many important biological processes.Progress in our understanding of the fundamental physics of conjugated polymers, which provides a crucial underpinning to the technological applications, has also been large. This progress has been driven by experimental, theoretical and computational developments.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. The strategy is to develop a theoretical understanding of the character of excited states by solving progressively more realistic and sophisticated model Hamiltonians. It then relates these properties to experimental observations in real systems, particularly to linear polyenes and the phenyl-based light emitting polymers.A number of important optical and electronic processes in conjugated polymers are also described, including non-linear optical properties, exciton transfer and migration, and dispersion interactions.There are numerous appendices containing more detailed technical explanations of the core material.