Studies on the electronic properties of conjugated polymers and low molecular weight organic solids have been of increasing interest in recent years. This book is organized into two parts dedicated to these two classes of materials. For each part a general introductory review provides background knowledge of the language and of the main points required for understanding the books contents. The reviews that follow provide a more complete understanding of the underlying physics of the materials through discussion of the interconnected topics. Theoretical concepts, models and methods are overviewed; this is used to support the explanation of the physical and chemical properties of these materials. The presentation of selected aspects of experimental research greatly contributes to the basic understanding of organic electronic materials.