Ballast water discharges have historically been a major source of nonindigenous species introductions to marine, estuarine, and freshwater ecosystems in the United States. Aquatic organisms may be discharged when the vessel discharges ballast tanks that contains such species, or when the vessel adds ballast water to tanks that contain such organisms in the residual water or sediment in the tank, and later discharges that mixture. When such organisms in ballast tanks are transported between waterbodies and discharged, the organisms may have the potential to establish viable new populations of species in waterbodies to which they are not native. Potentially, this introduction of nonnative aquatic nuisance species (ANS) via ballast water discharge can cause significant economic and ecological damage. Ballast water is necessary for the safe operation of the vast majority of larger vessels to assist with vessel draft, trim, and stability. Almost all large vessels have ballast tanks, pumps, piping and other equipment dedicated to this purpose. In lieu of or in addition to such dedicated systems, some vessels may also carry ballast water in otherwise empty cargo holds. Ballast water is typically drawn in from, and discharged directly to, ambient waters. The ballast water discharge rate and constituent concentrations of ballast water from vessels will vary by vessel type, ballast tank capacity, type of deballasting equipment, the quality of the ambient water from which the ballast water is drawn, the amount and quality of residual water or sediment in the tank, the efficacy of any ballast water management practices and/or ballast water treatment employed, and other factors. Vessels may discharge anywhere from less than a hundred cubic meters of ballast to tens of thousands of cubic meters.