Electrocatalytic water splitting driven by renewable energy input to produce clean hydrogen (H₂) has been widely considered a prospective approach for a future hydrogen-based society. However, the development of industrial alkaline water electrolyzers is hindered due to their unfavorable thermodynamics with high overpotential for delivering the whole process, caused by sluggish kinetics involving four-electron transfer. Further exploration of water electrolysis with low energy consumption and high efficiency is urgently required to meet the ever-growing energy storage and portfolio demands. This review emphasizes the strategies proposed thus far to pursue high-efficiency water electrolysis systems, including from the aspects of electrocatalysts (from monofunctional to bifunctional), electrode engineering (from powdery to self-supported), energy sources (from nonrenewable to renewable), electrolytes (from pure to hybrid), and cell configurations (from integrated to decoupled). Critical appraisals of the pivotal electrochemistry are highlighted to address the challenges in elevating the overall efficiency of water splitting. Finally, valuable insights for the future development directions and bottlenecks of advanced, sustainable, and high-efficiency water splitting systems are outlined.