To understand further, and to utilize, the interactions across protein-protein interfaces, we carried out an analysis of the hydrogen bonds and of the salt bridges in a collection of 319 non-redundant protein-protein interfaces derived from high-quality X-ray structures. We found that the geometry of the hydrogen bonds across protein interfaces is generally less optimal and has a wider distribution than typically observed within the chains. This difference originates from the more hydrophilic side chains buried in the binding interface than in the folded monomer interior. Protein folding differs from protein binding. Whereas in folding practically all degrees of freedom are available to the chain to attain its optimal configuration, this is not the case for rigid binding, where the protein molecules are already folded, with only six degrees of translational and rotational freedom available to the chains to achieve their most favorable bound configuration. These constraints enforce many polar/charged residues buried in the interface to form weak hydrogen bonds with protein atoms, rather than strongly hydrogen bonding to the solvent. Since interfacial hydrogen bonds are weaker than the intra-chain ones to compete with the binding of water, more water molecules are involved in bridging hydrogen bond networks across the protein interface than in the protein interior. Interfacial water molecules both mediate non-complementary donor-donor or acceptor-acceptor pairs, and connect non-optimally oriented donor-acceptor pairs. These differences between the interfacial hydrogen bonding patterns and the intra-chain ones further substantiate the notion that protein complexes formed by rigid binding may be far away from the global minimum conformations. Moreover, we summarize the pattern of charge complementarity and of the conservation of hydrogen bond network across binding interfaces. We further illustrate the utility of this study in understanding the specificity of protein-protein associations, and hence in docking prediction and molecular (inhibitor) design.