Electrochemical energy conversion holds the promise to be a cornerstone in a society driven by renewable energy sources. The electrochemical CO2 and CO reduction can enable sustainable production of fuels and chemicals. The research field has exploded in the last decade, giving rise to multiple publications trying to understand and improve the electrochemical CO2 and CO reduction reaction. In this thesis, I investigate the electrochemical CO2 and CO reduction using state of the art density functional theory (DFT) simulations. The reaction is investigated using binding energies of intermediates and electrolyte effects are explored by electrochemical interface simulations. Interface simulations are constructed by the metal surface, the intermediates and the electrolyte. The electrochemical CO2 and CO reduction are immensely difficult to approach, as multiple reaction intermediates and pathways are possible. This work primarily focuses on a direct comparison between experimental trend studies and calculated intermediate binding energies (descriptors). For comparison, metals and metal-nitrogen-carbon catalyst are investigated, which allows for a descriptor approach. It is found that the binding energy of CO and H form two universal descriptors for the reaction. These descriptors give insight into the product distribution which can be obtained from a CO2 reduction reaction experiment with a catalyst. This simple framework enables the possibilities of screening new catalysts for a wanted product selectivity. The electrochemical interface is investigated for the Cu(111), Cu(100) and Cu(110) facets by ab initio molecular dynamics (AIMD) of electrolytes in contact with the surfaces. These simulations reveal that some intermediates and anions are strongly stabilized by an aqueous electrolyte. The stabilization is used to explain experimental electrochemical CO reduction reaction results on Cu(100) in different electrolytes. Further, the interface simulations allows the derivation of ab initio cyclic voltammetry (CVs) in different electrolytes and pH. The thesis grasps a complex reaction in a simple descriptor framework and it provide tools to unravel reaction and CV dependencies with respect to the electrolyte effects.