The geometry and electronic structure of the amino group in aniline and its derivatives are very sensitive to both intramolecular interactions such as substituent effects and intermolecular ones such as H-bonding. An analysis of experimental geometries retrieved from the CSD base and computational modeling of aniline and its derivatives and their H-bonded complexes by use of B3LYP/6-311+G** and MP2/aug-cc-pVDZ showed that the degree of pyramidalization of the amino group depends on H-bonding, which exists in two forms, (i) NH center dot center dot center dot B (base) and (ii) N center dot center dot center dot HB (Bronsted acid), both of which affect the shape of the NH2 group. The effect may be significantly enhanced by a substituent through resonance interaction from electron-attracting substituents. The NH center dot center dot center dot B interactions lead to a substantial planarization of the group, whereas N center dot center dot center dot HB interactions do not. The natural bond orbital analysis allowed the authors to show that the changes in occupancy of the "lone pair" orbital and in geometry parameters describing pyramidalization of the group depend on the substituent constants.