This presentation explores heterogeneous electrochemical processes at electrode interfaces, with an emphasis on enabling clean energy conversion, storage, and utilization. A key challenge in this field is the development of high-performance inorganic electrocatalysts capable of maintaining sustained activity under operando conditions. Overcoming this challenge requires a fundamental understanding of how the structural, crystalline, and electronic characteristics of sub-nanoscale active sites evolve during real-time operation. To address this need, we apply advanced in-situ characterization techniques, including X-ray Absorption Spectroscopy (XAS), X-ray Diffraction (XRD), and Raman spectroscopy, to investigate the dynamic behavior of transition-metal-based catalysts (Mn, Fe, Co, Ni, Cu, Ru, Bi, Ag, and Ir). These studies span key electrochemical systems, including water and carbon dioxide electrolyzers, rechargeable zinc–air batteries, hydrogen fuel cells, and the electrochemical hydrogenation of biomass-derived molecules. Our results elucidate the transformation of catalyst precursors into sub-nanoscale active states and reveal direct correlations between structural and electronic modulation and catalytic performance. Notably, enhanced activity is linked to electronic interactions with supporting substrates and the distinctive chemical environments arising from sub-nanoscale architectures. Collectively, these findings establish a mechanistic framework for the rational design of durable, high-efficiency electrocatalysts for sustainable energy technologies.