With their ability to convert chemical energy of fuels directly into electrical power or reversibly store electrical energy, systems such as fuel cells and lithium ion batteries are of great importance in managing energy use. In these electrochemical energy conversion and storage (EECS) systems, controlled electrochemical redox reactions generate or store the electrical energy, ideally under conditions that avoid or kinetically suppress side reactions. A comprehensive understanding of electrode reactions is critical for the exploration and optimization of electrode materials and is therefore the key issue for developing advanced EECS systems. Based on its fingerprint and surface selection rules, electrochemical in-situ FTIR spectroscopy (in-situ FTIRS) can provide real-time information about the chemical nature of adsorbates and solution species as well as intermediate/product species involved in the electrochemical reactions. These unique features make this technique well-suited for insitu studies of EECS. In this Account, we review the characterization of electrode materials and the investigation of interfacial reaction processes involved in EECS systems by using state-of-the-art in-situ FTIR reflection technologies, primarily with an external configuration. We introduce the application of in-situ FTIRS to EECS systems and describe relevant technologies including in-situ microscope FTIRS, in-situ time-resolved FTIRS, and the combinatorial FTIRS approach. We focus first on the in-situ steady-state and time-resolved FTIRS studies on the electrooxidation of small organic molecules. Next, we review the characterization of electrocatalysts through the IR properties of nanomaterials, such as abnormal IR effects (AIREs) and surface enhanced infrared absorption (SEIRA). Finally, we introduce the application of in-situ FTIRS to demonstrate the decomposition of electrolyte and (de)lithiation processes involved in lithium ion batteries. The body of work summarized here has substantially advanced the knowledge of electrode processes and represents the forefront in studies of EECS at the molecular level.