In spectroscopy and diffraction methods, the signatures of catalytically active sites are often submerged by the contribution of spectator species. In some cases, the signals may also superimpose with each other, hindering proper peak identification. Rationalizing a reaction pathway becomes very challenging, if not impossible, under these circumstances. Accordingly, the implementation of transient, dynamic methods such as modulation/modulated excitation spectroscopy (MES) can improve the analysis of these signals. In MES, the catalyst sample is subjected to periodic changes in the environment (e.g., reactant concentration) that stimulate the active species periodically while spectra or diffractograms are recorded with sufficient time resolution. Combined with phase-sensitive detection (PSD) analysis, this approach selectively enhances the signals of the responsive (and possibly active) species and at the same time attenuates the contribution from the spectator and static species. Overall, this results in increased analytical capabilities irrespective of the type of spectroscopy or diffraction technique that is used for the experiment. In this chapter, we introduce the basic concepts of MES, discuss the theory of PSD, and provide general guidelines that are useful for whoever encounters PSD data for the first time.