During the last decade the advances in molecular biology have increasingly emphasized the importance of the hydrogen bond in biological interactions. Hydrogen-bonding in assemblages of biological molecules examined from the structural point of view is a main focus of the present book. Hydrogen-bonding is thereby defined in its broadest sense, as many attractive interactions involving a covalently bonded hydrogen atom. The concept of the hydrogen bond structure is developed, as is the role of cooperativity, in stabilizing particular structures and hydrogen-bonding dynamics. The information presented is based on the extraordinary increase in quantitative structural data that has become available from X-ray and neutron diffraction studies of crystal structures involving small and large biological molecules. Included are structural data of particular types of hydrogen bonds as well as data for different types of molecules, such as carbohydrates, purines and pyrimidines, nucleosides and nucleotides, ices and hydrates, cyclodextrins, nucleic acids and proteins.