Hydrogen energy, a new type of clean and efficient energy, has assumed precedence in decarbonizing and building a sustainable carbon-neutral economy. Recently, hydrogen production from water splitting has seen considerable advancements owing to its advantages such as zero carbon emissions, safety, and high product purity. To overcome the large energy barrier and high cost of water splitting, numerous efficient electrocatalysts have been designed and reported. However, various difficulties in promoting the industrialization of electrocatalytic water splitting remain. Further, as high-performance electrocatalysts that satisfy industrial requirements are urgently needed, a better understanding of water-splitting systems is required. In this paper, the latest progress in water electrolysis is reviewed, and experimental evidence from in situ/operando spectroscopic surveys and computational analyses is summarized to present a mechanistic understanding of hydrogen and oxygen evolution reactions. Furthermore, some promising strategies, including alloying, morphological engineering, interface construction, defect engineering, and strain engineering for designing and synthesizing electrocatalysts are highlighted. We believe that this review will provide a knowledge-guided design in fundamental science and further inspire technical engineering developments for constructing efficient electrocatalysts for water splitting.