MoS 2 is a promising low-cost catalyst for the hydrogen evolution reaction (HER). However, the nature of the active sites remains debated. By taking the electrochemical potential explicitly into account using grand-canonical density functional theory (DFT) in combination with the linearized Poisson-Boltzmann equation, we herein revisit the active sites of 2H-MoS 2. In addition to the well-known catalytically active edge sites, also specific point-defects on the otherwise inert basal plane provide highly active sites for HER. Given that HER takes place in water, we also assess the reactivity of these active sites with respect to H 2 O. The thermodynamics of proton reduction as a function of the electrochemical potential reveals that four edge sites and three basal plane defects feature thermodynamic over-potentials below 0.2 V. In contrast to current proposals, many of these active sites involve adsorbed OH. The results demonstrate that even though H 2 O and OH block "active" sites, HER can also occur on these "blocked" sites, 1 reducing protons on surface OH/H 2 O entities. As a consequence, our results revise the active sites, highlighting the so far overlooked need to take the liquid component (H 2 O) of the functional interface into account when considering the stability and activity of the various active sites.