Based on density functional theory calculations, kinetic measurements, microkinetic and Monte Carlo simulations, thermogravimetric analysis (TGA) experiments, extended X-ray absorption spectroscopy (EXAFS) measurements, and experimental results from the literature, we present a detailed and comprehensive mechanistic picture of the steam reforming process on a Ni catalyst. The complete potential energy diagram for the full reaction is presented and compared to experiments. Structures, energies of all reaction intermediates, and activation barriers are identified. We conclude that there are at least two kinds of active sites with different reactivities for steam reforming: a more active one associated with defect (step) sites, and a less active one associated with close-packed facets. It is further suggested that the nucleation of graphite is initiated at the step sites. We show that additives like potassium, sulfur, and gold preferentially bind to the step sites of Ni, and on this basis it is suggested that the promotion by these additives in terms of a higher tolerance toward graphite formation involves the same basic mechanism: blocking of the step sites.