Pathways leading to the formation of complex organic molecules are described. Gas-phase processes that may build large carbon-chain species in cold molecular clouds are summarised. Catalytic reactions on grain surfaces can lead to a large variety of organic species, and model calculations of mantle formation by atom additions to multiply-bonded molecules are presented. The subsequent desorption of these mixed molecular ices can initiate a distinctive organic chemistry via ion-molecule pathways. The predictions of this theory are briefly compared with observations to show how possible organic formation pathways in the interstellar medium may be constrained.