Over the past decade, numerous non-PGM-based catalysts have been explored, demonstrating remarkable catalytic activities for water electrolysis, i.e. alloys, chalcogenides, coordination polymers, intermetallics, oxides, (oxy)hydroxides, and pnictides, et al. Among these catalysts, their high electrical conductivity and admirable catalytic kinetics render transition metal chalcogenides (TMCs) attractive candidates for overall water electrolysis. Unfortunately, TMCs suffer from unfavorable thermodynamic stability compared to conventional transition metal oxides. Under harsh operational conditions, one would expect that the ongoing reaction proceeds along with the selfoxidation of TMCs, raising a great challenge for identifying the true catalytically active species/sites and for understanding the underlying catalytic reaction mechanisms. In such cases, advanced in situ/operando characterization techniques have emerged as powerful tools to address these fundamental challenges. For instance, CoSe2, as a representative TMCbased material, shows great promise for catalyzing both the HER and OER due to its unique structural properties and electronic configurations. Previous studies have revealed that the OER CoSe2 catalysts undergo significant phase transformations during the reaction process, in which in situ-generated CoOOH-related species serve as a prerequisite for triggering the OER. However, the open question, "Do the oxidized anionic components play a role in the OER?" remains to be elucidated. In sharp contrast, CoSe2 is generally investigated as a robust material to catalyze the HER, and the dynamics of its catalytically active species and sites during the HER remain to be fully understood. Notably, two main crystal phases, namely the orthorhombic phase (o-CoSe2) with a marcasite-type structure and the cubic phase (c-CoSe2) with a pyrite-type structure, commonly exist in CoSe2. Structurally, both phases feature similar [CoSe6] octahedral motifs connected via characteristic Se-Se pairs. It has been demonstrated that o-CoSe2 exhibits better HER activities compared to c-CoSe2 under acidic conditions, whereas the opposite trend is observed in alkaline media. This raises several fundamental questions to be answered, such as "Where does the intrinsic HER activity of CoSe2 catalysts originate?", "What are the true catalytically active species and sites in CoSe2 catalysts during the HER?", and "How does phase transformation influence the intrinsic HER activities?". In this sense, operando monitoring of CoSe2 catalysts for the HER in different media is essential. Furthermore, the crucial role of phase engineering and oxidized anionic residues in the OER must also be clearly elucidated.