[en] This paper focuses on the problems pertaining to energy and phase randomization in large molecules, which include: electronic dephasing, vibrational dephasing, and population and phase changes in multi-level systems. The author describes how laser spectroscopy can be used to probe dynamics that are relevant to the nature of the state excited with light, the separation of intra- and intermolecular effects and what determines the dephasing of large molecules. A brief description of the origin of dephasing is presented