Typically,a Lewis acid and a Lewis base can react with each other and form a classical Lewis adduct.The neutralization reaction can however be prevented by ligating the acid and base with bulky substituents and the resulting complex is known as a "frustrated Lewis pair" (FLP).Since the Lewis acid and base reactivity remains in the formed complex,FLPs can display interesting chemical activities,with promising applications in catalysis.For example,FLPs were shown to function as the first metal-free catalyst for molecular hydrogen activation.This,and other recent applications of FLPs,have opened a new thriving research field.In this short-review,we recapitulate the computational and experimental studies of the H2 activation by FLPs.We discuss the thus-far uncovered mechanistic aspects,including pre-organization of FLPs,the reaction paths for the activation,the polarization of H--H bond and other factors affecting the reactivity.We aim to provide a rather complete mechanistic picture of the H2 activation by FLPs,which has been under debate for decades since the first discovery of FLPs.This review is meant as a starting point for future studies and a guideline for industrial applications.