Volume II/19 Molecular Constants mostly from Microwave, Molecular Beam and Sub-Doppler Laser Spectroscopy is a supplement to the earlier volumes II/4, II/6 and II/14. The slight change in the title reflects current trends in the applications of spectroscopic methods. Together, the four volumes collect all the critically evaluated data published up to 1989/90 on constants of free molecules obtained by classical and modern spectroscopy. The present Subvolume II/19 contains high resolution spectroscopic results for freely rotating paramagnetic and ionic molecular species and in Part d3 a substance index for all subvolumes of the volume II/19.