With the development of modern instruments and theories, a considerable amount of spectroscopic informationis being permanently collected on molecules. The infrared, in particular, is seeing extraordinary activities. Using Fourier transform interferometers and infrared lasers, accurate data are measured often with extreme sensitivity. These data are also analyzed and precise molecular parameters determined. Volume II/20, “Molecular Constants Mostly from Infrared Spectroscopy”, is a recent Landolt-Börnstein publication series bringing together these results. It is made up of several volumes (A, B, C, D) with comprehensive compilation of critically evaluated molecular constants of diatomic (A), linear triatomic (B); nonlinear triatomic (C), and other polyatomic (D) molecules. Subvolume II/20C1 is devoted to H₂O.