Understanding reaction mechanisms in photo- and electro-catalysis is fundamentally limited by the difficulty of directly observing key intermediates at catalyst interfaces. In this talk, I will present recent advances in operando and time-resolved spectroscopic analysis for elucidating interfacial processes in energy conversion reactions, including CO2 reduction, oxygen reduction, ammonia oxidation, and nitrogen transformation. A particular emphasis will be placed on operando attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) and in situ FT-IR spectroscopy, which enable real-time monitoring of surface-bound intermediates and isotopically labeled products under realistic reaction conditions. Time-resolved measurements provide critical insights into the kinetic relevance of intermediates and their coupling with charge transfer dynamics. Through representative case studies, I will demonstrate how operando spectroscopic insights link catalyst structure, defect engineering, and interfacial chemistry to activity, selectivity, and stability, offering practical guidelines for the rational design of next-generation photo- and electro-catalysts.