Polymeric insulators, semiconductors, metals and ionic conductors, their electronic structure and properties. Definitions and properties of quasi-particles, like excitons, polarons and solitons. The processes of optical absorption, fluorescence, non-radiative optical transitions, electron transfer, charge carrier generation and transport, charge photogeneration and injection, charge trapping, electroluminescence and non-linear optics are discussed. From the application point of view, the function and basic principles of polymeric electronic devices, such as photodetectors, solar cells, electroluminescent diodes and displays, rectifying elements, transistors and non-linear optical elements, are explained in detail. Also, the applications of electronically active polymers in new technologies are mentioned. From the chemical point of view, examples of the relations of molecular structure and electric properties, types of basic chemical syntheses and modifications of polymer materials are discussed.