We present a procedure for partitioning the Hartree-Fock self-consistent-field (SCF) interaction energy into electrostatic, charge transfer, and deformation components. The natural bond orbital (NBO) approach of Weinhold and co-workers is employed to construct intermediate supermolecule and fragment wave functions that satisfy the Pauli exclusion principle, thereby avoiding the principal deficiency of the popular Kitaura-Morokuma energy decomposition scheme. The function counterpoise method of Boys and Bernardi enters the procedure naturally, providing an estimate of basis set superposition error (BSSE). We find that the energy components exhibit little basis set dependence when BSSE is small. Applications are presented for several representative molecular and ion complexes : the weak hydrogen bond of the water dimer, the strong ionic interaction of the alkali metal hydrides, and the moderate donor-acceptor interactions of BH3NH3 and BH3CO. Electrostatic interaction dominates the long-range region of the potential energy surface and charge transfer is strongly stabilizing for fragments within van der Waals contact. The repulsive interaction in the short range region of the potential arises from deformation as the fragment wave functions distort to avoid significant interpenetration.