The accuracy of present-day X-ray and neutron diffraction studies is such that root-mean-square vibrational amplitudes of atoms in complex structures can be determined to an accuracy of 0.01 Å or better. These amplitudes are usually in agreement with spectroscopically derived frequencies where these are available. With the aid of suitable models, amplitudes of rigid body rotations, translations, screw motions, and of internal rotations may be obtained. Such information is invaluable in the interpretation of spectra from complex crystals and in the understanding of intermolecular forces. Several examples from recent work at Brookhaven will be given.