With the advent of modern instruments and theories, a considerable amount of spectroscopic information has been collected on molecules during this last decade. The infrared, in particular, has seen extraordinary activity. Using Fourier transform interferometers and infrared lasers, accurate data have been measured, often with extreme sensitivity. These data have also been analyzed and accurate molecular parameters determined. Volume II/20 "Molecular Contants mostly from Infrared Spectroscopy" is a recent Landolt-Börnstein publication series. It is made up of several subvolumes (A, B, C, D) with comprehensive compilation of critically evaluated molecular constants of diatomic (A), linear triatomic (B), other triatomic (C) and other polyatomic (D) molecules.