Delocalization indices, as defined in the atoms in molecules theory, have been calculated between hydrogen-bonded atoms in 20 molecular complexes that are formed between several H-donor and acceptor molecules. In general, the delocalization index associated to an intermolecular hydrogen bond depends on the interaction energy of the complex, but also on the nature of the H-donor and acceptor atoms. The intermolecular delocalization index appears to be strongly correlated to the orbital interaction energy term as obtained from an energy decomposition scheme based on conceptual Kohn-Sham theory. Both the atoms in molecules theory and the energy decomposition analysis allow for a characterization of the hydrogen bonds in these complexes. In general, both kinds of analysis reveal that hydrogen bonding in these systems is not an essentially electrostatic interaction.