However, TMSs are generally thermodynamically unstable when compared to conventional transition metal oxides. Under the harsh oxidation conditions of water electrolysis, one would expect that the oxidation of sulfides occurs along with the running anodic reactions. Therefore, precisely uncovering the nature of the true catalytically active species and sites of TMSs during the reaction conditions is of utmost importance to guide the design of the best TMS catalysts toward practical applications. To address these long-standing challenges, new operando monitoring studies of TMSs are required. Recently, in situ/operando spectroscopic or microscopic studies shed light on the OER mechanisms of TMSs. The results show that an irreversible structural transformation from sulfides to their corresponding oxides/hydroxides is a prerequisite to trigger the OER. Interestingly, these in situ formed oxides/hydroxides often offer better OER activities compared to conventionally synthesized ones, yet remain poorly understood. In sharp contrast, TMSs frequently emerged as robust catalysts to trigger the HER. Theoretical studies reveal that the HER kinetics of TMSs proceed through the water dissociation and the adsorption/desorption of H* intermediate at the active sites, but these theoretical insights cannot fully explain or predict their HER activities in different pH media, i.e. acidic and alkaline conditions lead to differences in the observed HER activity. Further, it has to be taken into account that in some OER studies structural transformation of TMSs was even observed at open circuit potential, which is in line with the notably lower equilibrium of S 2-/SO4 2-(0.15 V vs. RHE) under aqueous conditions. On this basis, one can expect that similar oxidation processes from S 2-to SOx -can not be fully excluded in TMSs during the HER. Indeed, fundamental questions as to "What are the true catalytically active species of sulfide HER catalysts?" need to be answered. To the best of our knowledge, employing in situ/operando techniques to understand the HER kinetics of sulfide-related catalysts has not been fully established up to now.