This is a revised and expanded version of an article that appeared in the 2nd edition of the Encyclopedia of Physics, R.G. Lerner and G.L. Trigg, eds. Topics covered include Infrared, Raman, rotational, vibration-rotational, and electronic spectra, as well as matrix isolation, liquid helium droplet and other low-temperature, ZEKE, single molecule, and terahertz spectroscopies. Also addressed are issues of molecular structures of free molecules and their van der Waals and hydrogen-bonded complexes, as well as the spectroscopic studies of the earth's and planetary atmospheres, molecular clouds, circumstellar atmospheres and that of the interstellar medium and standard reference data.