More than 50 [NbxCy](+) clusters have been observed in the gas phase. We report a density functional investigation of 100 isomers of 28 of the most prominent of these clusters, ranging in size from [Nb2C2] to [Nb12C20] and [Nb14C16], calculated as both neutral and monocation species. Vertical ionization energies and normalized binding energies are provided for all isomers, and the most probable structures of the 28 [NbxCy] clusters are assigned. The species [Nb8C12] and [Nb14C13] have the same structures (Td and Oh, respectively) as the Ti analogues. The calculations and interpretations generate a broad picture of the geometric structures, electronic structures, and observed reactivities of Nb,C, clusters. The relevant structural principles and the key substructural components have been defined. At the local level, Nb-C bonding is the principal source of stability together with C-C bonding where it occurs, whereas Nb-Nb bonding energy is less important but still significant. Cradle-motif Nb4C2 moieties are important, but not if they are strongly distorted. The 3-fold wheel substructure and the spire (3-fold or 3-fold) are key substructures involving C-2 Fused cubanoid units as occur in the nanocrystallites are the main containers of isolated C atoms and are usually slightly distorted. Substitution of Ct in place of C in a nanocrystallite occurs in some NbxCy clusters that are slightly C-rich. Fusion of wheels or spires onto nanocrystallite sections occurs with increasing C content, and the most C-rich clusters contain C-2 units only, but not longer C-n chains.