Renewable electricity-driven water electrolysis offers a promising route to low-emission hydrogen, yet exceptional laboratory catalyst performance does not necessarily translate into efficient, durable, and scalable electrolyzer operation. This review examines nanostructured electrocatalysts across interconnected length scales, linking atomic-level electronic structure and adsorption energetics with the potential dependent active phase, electrode transport processes, and complete device integration. Noble metal catalysts, nickel–molybdenum systems, molybdenum disulfide, nickel–iron oxyhydroxides, reconstructing precatalysts, atomically dispersed sites, and compositionally complex materials are critically assessed according to the evidence needed to identify active sites and validate long-term stability. Particular attention is given to alkaline water electrolysis, proton-exchange membrane water electrolysis, anion-exchange membrane water electrolysis, oxygen ion solid oxide electrolysis, and proton-conducting ceramic electrolysis. The review shows that apparent activity can be overstated by surface roughness, transient reconstruction, electrolyte purity, or low catalyst loading, while practical deployment is limited by catalyst layer uniformity, membrane compatibility, ion transport, water supply, gas removal, dissolution, thermal management, dynamic loading, and manufacturing constraints. A unified translation framework is proposed to distinguish mechanistic promise from device-relevant progress and guide the development of efficient, durable, resource-conscious, and verifiably low-emission hydrogen production at scale.