The lengthening of the covalent X-H bond in heteronuclear hydrogen bonds is quantified from neutron crystal structures for combinations of the donors N-H and O-H with the accepters O, N, S, and Cl-. For N-H O hydrogen bonds, only high-precision low-temperature crystal structures are used. For the other hydrogen bond types, there are too few neutron crystal structures available to allow restriction to low-temperature data. For all donor/acceptor combinations studied, systematic lengthening of the X-H bond due to the hydrogen bond interaction is definitely confirmed, and this includes the relatively weak sulfur accepters. The experimental data are discussed in the frame of the valence model of the hydrogen bond, and on this basis, the geometries of a number of hydrogen bonds are predicted for which only X-ray crystal structures have been published.