Contents 1 Basic concepts 2 The localized electron: magnetic properties 3 The moving electron: electrical properties 4 The excited electron: photophysical properties 5 The mastered electron: molecular electronics The purpose of the book is to provide the reader with essential keys to understand in an unified way the rapidly expanding field of molecular materials and devices: electronic structure and bonding, magnetic, electrical and photophysical properties and therefore the mastering of electrons i.e. molecular electronics. The chemist will find how basic quantum concepts allows to understand the relations between structures, electronic structures and properties of molecular entities and assemblies and then to design new molecules and materials. Physicists and engineers will realize how the molecular world fits with their demands of systems flexible enough to check theories or provide original solutions to exciting new scientific and technological challenges. The non specialist will discover why and how molecules invite themselves in electronics at the ultimate, subnanosize, level. Such a unified view is unique and will benefit to undergraduate and graduate students in chemistry, materials science, engineering and to researchers willing to have a simple introduction for entering the world of molecular materials.