This pelper-is deciicated to Professor Sigeru Hl~zir~ugn or1 the occasiotz of his 65th birthday R. F. W. BADER and D. A. LECARE. Can. J . Cheni. 70, 657 (1992). The energies, geometries, and charge distributions of the closo-, tlido-. and arachrro-boranes and closo-carboranes are determined in SCF calculations using a contracted (9s,5/2) basis set of Huzinaga, supplemented with polarization functions. Good agreement is obtained between the experimental and calculated geometries. Molecular structures are assigned on the basis of the bond paths defined by the topology of the charge density using the theory of atoms in molecules. This theory is used to characterize the atomic interactions and to account for the stabilities of these electron-deficient systems. It is shown that their stability is a consequence of the delocalization of charge over the surfaces of the three-and four-membered rings of atoms that result from the formation of bonds of reduced order, a delocalization that is itself essential to the fonnation of the ring bonds. The Laplacian of the charge density is used to predict the sites of nucleophilic and electrophilic attack in these molecules, predictions that are in agreement with known experimental findings.