Density functional theory (DFT) by itself is insufficient to model electrochemical reactions, because the interface is too large, and there is no satisfactory way to incorporate the electrode potential. In our group we have developed a theory of electrocatalysis, which combines DFT with our model for electrochemical electron transfer, and thereby avoids these difficulties. Our theory explains how a metal d band situated near the Fermi level can lower the energy of activation for a charge transfer reaction. An explicit application to the hydrogen evolution reaction gives results that agree very well with experimental data obtained both on plain and on nanostructured electrodes. Finally, we outline how our method can be extended to other reactions and present first results for the adsorption of OH on Pt(111).