Modulation excitation spectroscopy (MES) allows sensitive and selective detection and monitoring of the dynamic behavior of species directly involved in a reaction. The method, combined with proper in situ spectroscopy, is powerful for elucidating complex systems and noisy data as often encountered in heterogeneous catalytic reactions at solid–liquid and solid–gas interfaces under working conditions. The theoretical principle and actual data processing of MES are explained in detail. Periodic perturbation of the system by an external parameter, such as concentration and temperature, is utilized as stimulation in MES. The influence of stimulation shape upon response analysis is explained. Furthermore, an illustrative example of MES, enantioselective hydrogenation at a solid-liquid interface, is presented.