The covalent nature of interactions within various hydrogen bonded molecular aggregates has been characterized by the two entirely different computational methods: Bader analysis of the electron density and variation-perturbation partitioning of the intermolecular interaction energy. Analysis of 34 complexes representing different types of hydrogen bonds indicates that the proton-acceptor distance approximately 1.8 A and the ratio of delocalization and electrostatic terms approximately 0.45 constitutes approximately a borderline between covalent and noncovalent hydrogen bonds. The latter ratio could be used to characterize quantitatively the degree of the covalent nature of transition state interactions with active site residues, a quantity essential for an enzyme catalytic activity.