Hydrogen embrittlement (HE) is one of the most dangerous yet most elusive embrittlement problems in metallic materi-als.Advanced high-strength steels (AHSS) are particularly prone to HE,as evidenced by the serious degradation of their load-bearing capacity with the presence of typically only a few parts-per-million H.This strongly impedes their further development and application and could set an abrupt halt for the weight reduction strategies pursued globally in the auto-motive industry.It is thus important to understand the HE mechanisms in this material class,in order to develop effective H-resistant strategies.Here,we review the related research in this field,with the purpose to highlight the recent progress,and more importantly,the current challenges toward understanding the fundamental HE mechanisms in modern AHSS.The review starts with a brief introduction of current HE models,followed by an overview of the state-of-the-art micromechani-ca1 testing techniques dedicated for HE study.Finally,the reported HE phenomena in different types of AHSS are critically reviewed.Focuses are particularly placed on two representative multiphase steels,i.e.,ferrite-martensite dual-phase steels and ferrite-austenite medium-Mn steels,with the aim to highlight the multiple dimensions of complexity of HE mechanisms in complex AHSS.Based on this,open scientific questions and the critical challenges in this field are discussed to guide future research efforts.