This book is devoted to a general discussion about localization and delocalization in quantum chemistry. The first volume is concerned with molecules in their ground state. It is made of papers presented during the academic year 73-74 at an international seminar organized by some members of the 'Centre de Mecanique Ondulatoire Appli quee du C.N.R.S.' and some members of the 'Laboratoire de Chimie Quantique de l'Institut de Biologie Physico-Chimique'. It contains also reports of discussions which followed the presentation of invited papers. It is a 'forum' in which each expert gives his opinion on a work in progress. The volume is divided into four parts. The first one is a statistical analysis of the localizability of molecular electrons in the three-dimensional space. It contains an exposition of the basic ideas of the loge theory which provides a framework to do such an analysis. The second part is concerned with the separability of a molecular wave function and its expression in terms of localized elements. An exploration is made of the rela tionship between the localizability of electrons and the possibility of expressing the wave function in terms of localized orbitals. The third part is devoted to the partition of the energy in local contributions.