The development history of hydrogen internal combustion engines（H₂ICEs）around the world was sorted out, and the detailed analysis of physicochemical characteristics and flame propagation of hydrogen was conducted with an emphasis on the current research status of key technologies including hydrogen injection, abnormal combustion, NO_x emission, lubrication, safety, and engine performance. Driven by the global carbon neutrality, energy saving and emission reduction regulations, a significant progress has recently been made in hydrogen direct injection and its mixture preparation, hydrogen pre-ignition and detonation suppression, NO_x emission aftertreatment and dedicated lubricants for H₂ICEs . Under lean-burn conditions, the brake thermal efficiency of H₂ICEs has now exceeded 45%, and near-zero NO_x emission can be achieved. Finally, critical factors are outlined for the large-scale application of H₂ICEs, and the needs for further improvement in safety and reliability are highlighted. Notably, hydrogen direct injection and ultra-high supercharging technologies are identified as pivotal solutions for enhancing both power output and fuel economy of H₂ICEs.