Our progressive transition from a society energetically dependent on fossil fuels towards one relying on renewable sources requires novel, environmentally friendly energy conversion and storage concepts. Hydrogen is widely regarded as an energy carrier that could circumvent this need, particularly in sight of the foreseeable spread of fuel cell cars that would use this “renewable H2”. The latter would be produced using electrolyzers, which in their better established form cannot fulfill the targeted H2-price due to the low current densities ( ) associated to their liquid electrolyte. Alternatively, devices based on proton- and anion-exchange membranes are currently under development, and a new kind of co-electrolysis cell in which CO2 is reduced into hydrocarbons is also envisaged. Electrocatalysts play a crucial role in all of these systems, but the interplay between their surface and the reaction medium (in the so-called interface) is often overlooked in the quest towards better performance. With this motivation, this review discusses the current knowledge of the interfacial catalysis of the three (co-)electrolysis relevant reactions (i.e., the evolution of H2 and O2, and the reduction of CO2). From this, we identify pH and surface oxidation state as the key electrolyte- and surface-related parameters for which further understanding could lead to improved kinetics, and propose strategies for the tentative design of better electrocatalysts.