The aim of this book is to explain the unusual properties of both pure liquid water and simple aqueous solutions, in terms of the properties of single molecules and interactions among small numbers of water molecules. It is mostly the result of the author's own research spanning over 40 years in the field of aqueous solutions. An understanding of the properties of liquid water is a prelude to the understanding of the role of water in biological systems and for the evolvement of life. The book is targeted at anyone who is interested in the outstanding properties of water and its role in biological systems. It is addressed to both students and researchers in chemistry, physics and biology.