This book is intended for those who are interested in understanding the electronic structure and properties of polymers. The scope of the book is to provide the non-specialist reader with a comprehensive and unified description: (i) of quantum mechanical methods, mainly originating from quantum chemistry, to calculate the electronic properties of polymers, (ii) of their use for interpreting and predicting results in fields where the electronic structure is playing an important role, like the electrical conductivity and the non linear optical properties of conjugated polymers.It will also serve