We present an ab initio, quantum mechanical study of 18-crown-6 (18c6) and its interaction with the alkali metal cations Li+, Na+, K+, Rb+, and Cs+. Geometries, binding energies, and binding enthalpies are evaluated at the restricted Hartree-Fock (RHF) level using standard basis sets (3-21G and 6-31+G*) and relativistic effective core potentials. Electron correlation effects are determined at the MP2 level, and wave function analysis is performed by the natural bond orbital (NBO) and associated methods. The affinity of 18c6 for the alkali metal cations is quite strong (50-100 kcal mol(-1), depending on cation type), arising largely from the electrostatic (ionic) interaction of the cation with the nucleophilic ether backbone. Charge transfer (covalent bonding) contributions are somewhat less important, only 20-50% as strong as the electrostatic interaction. Agreement of the calculated binding enthalpies and experimentally determined quantities is rather poor. For example, the binding energy for K+/18c6 (-71.5 kcal mol(-1)) is about 30 kcal mol(-1) stronger than that determined by experiment, and it is not clear how to reconcile this difference. Our calculations clearly show that solvation effects strongly influence cation selectivity. Gas-phase 18c6 preferentially binds Li+, not K+ as found in aqueous environments. We show, however, that K+ selectivity is recovered when even a few waters of hydration are considered.