The Chesapeake Bay Environmental Model Package (CBEMP) was used to assess the environmental benefits of oyster restoration in Chesapeake Bay. The CBEMP consists of a coupled system of models including a three-dimensional hydrodynamic model, a three-dimensional eutrophication model, and a sediment diagenesis model. Restoration levels up to fifty times the 1994 base biomass were examined. Examination of results emphasized dissolved oxygen, chlorophyll concentration, and water clarity. Within Virginia, the improvement in summer, bottom-water dissolved oxygen at the maximum biomass investigated was 0.2 mg/L. Within Maryland, the improvement was doubled, more than 0.4 mg/L. Within Virginia, the range of oyster densities investigated reduced summer-average surface chlorophyll by up to 0.7 uig/L, roughly 10 percent of the 1994 base concentration. Corresponding reductions in Maryland were up to 2.3 uig/L, more than 25 percent of the 1994 base. Within Virginia, the range of oyster densities investigated reduced summer-average light attenuation by up to 8 percent, from 1.05 m-1 at base levels to 0.97 m-1 for a fifty-fold increase in oyster biomass. Following a pattern established for other benefits, improvements in Maryland exceeded Virginia. Summer-average light attenuation diminished by up to 13 percent, from 1.39 m-1 under base conditions to 1.21 m-1 for a fifty-fold increase in oyster biomass.