This book originated from a course which I developed for the Master's degree course in Molecular Engineering in Kyoto University. Most of the students had degrees in Chemistry and a limited experience of Physics and Mathematics. Since research in Molecular Engineering requires knowledge of some applications of solid state physics which are not treated in conventional physics texts it was necessary to devise a course which would build on their chemical background and enable them to read the contemporary literature of relevance to their research. I hope that this book will be found useful as a text for other advanced courses on material science for chemists. Molecular Engineering is concerned with the design and construction, at the molecular level, of materials which can fulfil specific functions. Thus the study of the forces between molecules and the influence of molecular shapes and electrostatic features on molecular properties are important. The mechanisms whereby, in the solid state, these produce cooperative effects, catalytic effects and abnormal electrical effects must be understood, at least qualitatively. The aim of this book has been to give insight into the mechanisms whereby molecules influence one another when they are close together.