When writing a book on as vast a subject as Hydrogen Bonding, a major problem is to circumscribe the subject matter. In this respect, we have adopted the crystallographer's point of view. We have focused our attention on the structure of the hydrogen-bonding patterns, as revealed by crystal structure analyses and described by the relative positions of atoms in three dimensions on the nanometer scale in the crystalline state. The molecules which are important in biological reactions have dimensions ranging from approximately 10 to 1000 A.1 The cohesive forces between them are such that the nearest neighbor interatomic distances between these molecules fall in the relative narrow range of 1.5 A for strong cohesive forces between atoms, to 3.5 A for the weak forces. The topology of the assemblages of biological molecules which take part in biological processes must therefore be studied on this scale.