The NATO Advanced Study Institute on Electronic Structure of Polymers and Molecular Crystals was held at the Facultes Universi taires de Namur (F.U.N.) from September 1st till September 14th, 1974. We wish to express our appreciation to the NATO Scientific Affairs Division whose generous support made this Institute possible and to the Facultes Universitaires de Namur and the Societe Chimique de Belgique which provided fellowships and travel grants to a number of students. This volume contains the main lectures about the basic principles of the field and about different recent developments of the theory of the electronic structure of polymers and molecular crystals. The school started with the presentation of the basic SCF-LCAO theory of the electronic structure of periodic polymers and molecular crystals (contributions by Ladik, Andre & Delhalle) showing how a combination of quantum chemical and solid state physical methods can provide band structures for these systems. The numerical aspects of these calculations were also discussed. Lectures by Mahan have shown how optical properties of molecular crystals can be interpreted on the basis of the exciton theory. Little has reviewed the present status of the hypothesis about excitonic superconductivity and the different approaches to synthesize a superconductive polymer. McCubbin in his first series of lectures has given a very precise group theoretical treatment of the symmetry properties of polymers. Atkins’ lectures have covered both the theoretical aspects of X-ray structure determination and its application to different polymers.