Since the publication of the previous volumes many new aspects of the physical and life sciences have been developed in which the properties of water play a dominant role. Although, according to its preface, Volume 5 was to be the last one of the treatise, these recent developments have led to a revision of that statement. The present volume and its companion, still in preparation, deal with topics that were already mentioned in the preface to Volume 5 as gaining in importance. The recent development of X-ray and, more particularly, neutron scattering techniques have led to studies of structure in aqueous solutions of electrolytes on the one hand, and to the role of water in protein structure and function on the other. Both these topics have reached a stage where reviews of the present state of knowledge are useful. The application of ab initio methods to calculations of hydration and conformation of small molecules has a longer history, but here again a critical summary is timely. The role of solvent effects in reaction kinetics and mechanisms should have had a place in Volume 2 of this treatise, but, as sometimes happens, the author who had taken on this task failed tQ live up to his promise. However, since 1972 the physical chemistry of mixed aqueous solvents has made considerable strides, so that the belated discussion of this topic (by a new author) is built on evidence that was not available at the time of publication of Volume 2.