The resonance-assisted hydrogen bond (RAHB) is a model of synergistic interplay between π-delocalization and hydrogen-bond (H-bond) strengthening originally introduced (Gilli, G.; Bellucci, F.; Ferretti, V.; Bertolasi, V. J. Am. Chem. Soc. 1989, 111, 1023; Bertolasi, V.; Gilli, P.; Ferretti, V.; Gilli, G. J. Am. Chem. Soc. 1991, 113, 4917) for explaining the abnormally strong intramolecular O−H···O bonds formed by the ···OC−CC−OH··· β-enolone fragment I which are typical of β-diketone enols. The applicability of this model to the intramolecular N−H···O hydrogen bonds formed by a number of heteroconjugated systems (···OC−CC−NH···, β-enaminones II; ···OC−CN−NH···, ketohydrazones III; and ···ON−CC−NH···, nitrosoenamines IV) is investigated. The X-ray crystal structures of five molecules which close a six-membered ring by an intramolecular N−H···O bond through the resonant ···OX−CX−NH··· (X = C, N) fragments II−IV are compared to those of two other molecules closing the same ring through the nonresonant ···OC...