This paper addresses questions important to the origin of optical dephasing and vibrational energy redistributions in molecules. Several laser techniques are discussed and three major findings are presented. These findings are related to (a) optical dephasing of molecules in the gas phase and in beams, (b) dephasing of high-energy vibrational overtone states of large molecules, and (c) energy randomization and quantum beats in large molecules (anthracene) excited by picosecond pulses and cooled by supersonic jet expansion. © 1982 American Chemical Society.