Hydrogen-bonded complexes involving sulfur bases are found to be quite different from the analogous oxygen complexes, both experimentally and in theoretical calculations. In general, hydrogen bonds to sulfur not only are weaker than those to oxygen but also show a marked preference for a more "perpendicular" direction of approach to the donor atom. Ab initio calculations at the MP2/6-311++G(d,p) level on the complexes of hydrogen fluoride with H2O, H2S, H2CO, and H2CS reproduce these differences, as does a search of structures in the Cambridge Crystallographic Data Base. We show that the Laplacian of the charge density del(2) rho predicts a qualitatively correct structure for all the systems considered, but gives poor quantitative predictions of hydrogen-bonding geometries. An analysis based upon Bader’s atoms-in-molecules theory rationalizes the differences between sulfur and oxygen hydrogen bonds. A treatment of the hydrogen bond which explicitly considers the contributions of atomic multipoles to the electrostatic energy has more success than del(2) rho in predicting H bond directionality. Hydrogen bond formation to oxygen is driven by charge-charge interactions, whereas with sulfur the stabilization arises principally from the interaction of the charge on the acidic hydrogen with the dipole and quadrupoles of sulfur.