Geometries of guanidine and eight of its N-imino derivatives (CH3, SiH3, OH, CN, F, Cl, CHO, and NO2) were calculated using ab initio molecular orbital techniques. MP2, MP4, and CISD geometries indicate that the guanidine molecule is pyramidal at amino groups and pi-conjugation through the amidine skeleton is modest. MP2 structures of the eight N-imino guanidines reveal that substituting the hydrogen atom of the imino nitrogen by a functional group leads to a concerted variation of the CN bond distances, Topological electron density analyses indicate that the structural changes calculated upon N-imino substitution are largely due to changes of conjugation onto the amidine pi-skeleton. Intermolecular hydrogen bonding involving the imino and amino groups is also found to affect the amidine geometry. Special attention has been paid in the work of 2-cyanoguanidine. The centrosymmetric dimer and a more extended hydrogen-banded complex have been proposed as simulation of the crystal environment and the respective ab initio geometries (HF/4-31G(d) and MP2/6-31G(d)) are discussed. The structural changes due to self-association have been rationalized supposing that the contribution of polar canonical forms increases upon hydrogen-bonding formation.