This book aims to understand the main aspects of non-covalent chemistry (mainly in the gas phase) and specifically compares the experimental and theoretical data available for non-covalent complexes and subsequent problems associated with this comparison. The book is authored by an experimentalist (KMD) and theorist (PH), and their main philosophy in writing together is that any book on non-covalent interactions cannot be limited either to theory or experiment. Both approaches are nowadays so closely connected that one cannot exist without the other and vice versa and their mutual connection provides the consistent description of non-covalent processes in our world. This book will be of great assistance to researchers engaged in both theoretical and experimental aspects of non-covalent bonding and in macro- and supermolecular chemistry.