Introduction.- Quantum Chemical Methods.- Rovibrational Spectroscopy and Structure of Diatomic Molecules.- Rotational Constants of a Polyatomic Molecule.- Equilibrium Structures of Semirigid Molecules from the Rotational Constants.- Structure of Nonrigid Molecules by Spectroscopic Methods.- Equilibrium Molecular Structure as Determined by Gas-phase Electron Diffraction.- Other Methods, Mainly for the X-H bond (X = C, N, O).- Database with Equilibrium...