Grass carp have effectively controlled submergent aquatic macrophytes in large reservoirs when stocked at densities up to 75 fish per vegetated hectare. Large systems pose special concerns relative to decisions on whether to stock grass carp. Multiple use, multiple agency involvement, and interstate jurisdictions complicate decisionmaking. Complex vegetation communities of large systems make results less predictable. Grass carp movements not only allow impacts on nontarget areas but also may result in incomplete control. Grass carp biology is understood well enough to predict effects on vegetation, but vegetation responses to grass carp are not predictable enough for vegetation to be managed rather than controlled. This paper discusses the potentials and limitations for grass carp in large, open systems based principally upon studies in Lake Conroe, Tex., where 45 percent of the 8,000–hectare reservoir was infested by submergent macrophytes.