Underlying Structure-Activity Correlations of 2D Layered Transition Metal Dichal | Physics TomorrowAbstract H ydrogen fuel is an ideal energy source to replace the traditional fossilfuels because of its high energy density and renewability.Elec trochemical water splitting is also regarded as a sustainable, cleaningand eco friendly method for hydrogen evolution reaction (HER), but acheaper, earth abundant and similarly efficient alternative to Pt as anHER catalyst cannot still be discovered. Re cently, 2D Transition MetalDichalcogenides (TMDs) are demonstrated to greatly enhance the HERactivity. Herein, our work provides an insight into the recent advancesin 2D TMDs based HER following the composition characterisationconstruction guideline. A fter the background introduction, severalresearch outputs based on 2D TMDs as well as the comprehensiveanalysis on the modulation strategies of 2D TMDs, for the purposes ofincreasing the active sites, improving the intrinsic activity and alteringthe el ectronic states. Finally, the future opportunities and challenges of2D TMDs electrocatalysts are briefly featured.