Electrified interfaces play a prime role in energy technologies, from batteries and capacitors to heterogeneous electrocatalysis. The atomistic understanding and modelling of these interfaces is challenging due to the structural complexity and the presence of the electrochemical potential. Including the potential explicitly in the quantum mechanical simulations is equivalent to simulating systems with a surface charge. For realistic relationships between the potential and the surface charge (i.e., the capacity), the solvent and counter charge need to be considered. The solvent and electrolyte description are limited by the computational power: either molecules and ions are included explicitly, but the phase-space sampling is at least 10 times too small to reach convergence or implicit solvent and electrolyte descriptions are adopted which suffer from a lack of realism. Both approaches suffer from a lack of validation against directly comparable experimental data. Furthermore, the limitations of density functional theory in terms of accuracy are critical for these metal/liquid interfaces. Nevertheless, the atomistic insight in electrocatalytic interfaces allow insights with unprecedented details. The joint theoretical and experimental efforts to design non-noble hydrogen evolution catalysts are discussed as an example for the success of theory to spur and accelerate experimental discoveries. Graphical/Visual Abstract and Caption