Atomistic simulation techniques are used to investigate the surface structure, stability and reactivity of pyrite. We introduce a potential model for FeS2 which reproduces experimental structural parameters, elastic constants and hydration energies of pyrite. We modeled the {100}, {110}, and {111} surfaces of pyrite and calculated the {100} surface to be the most stable and to show little surface relaxation, in agreement with experiment. The surfaces were hydrated by associative adsorption of water molecules which stabilized all three surfaces, especially the unstable {111} surface. The calculated adsorption energy of -47 kT mol(-1) for water on the {100} surface agrees well with an adsorption energy of -42 kJ mol(-1), determined for the stoichiometric (100) surface by temperature-programmed desorption.(1) Adsorption of water molecules at surface sites of lower coordination (four- or three- coordinated) showed increased reactivity of these sites. We calculated an increase in adsorption energy of 50-60 kJ mol(-1) per loss of bond. We next created stepped {100} planes in to model a more realistic {100} surface with one-dimensional defects. Four different steps were investigated in two orthogonal directions. Because of the asymmetry of the sulfur dimers, the geometry of the dimers on the edge showed the dimers either leaning forward (F-steps) or backward (B-steps) with respect to the {100} terrace. We used water molecules as a probe of the reactivity of the different surface sites. Corresponding adsorption sites (terrace, edge or below the step) on the F- and B-steps were found to have different reactivities toward water due to the different adsorption modes of the probe molecule. On the B-steps the increased reactivity of the low-coordinated edge iron atom toward water (approximately -70 kT mol(-1)) was outweighed by the network of interactions of the water molecule to atoms on terrace and step wall in the position below the step (-91 kJ mol(-1)). On the F-steps the four-coordinated edge site was calculated to be the most reactive adsorption site.