The Laurentian Great Lakes are the largest unfrozen reservoir of freshwater on earth, accounting for almost one-fifth of the worlds fresh surface water. They are vital to the economy of the Great Lakes region and to the quality of life of its residents, providing drinking water for more than 33 million people in Canada and the United States, supplying hydroelectric power, supporting industries, providing waterborne transportation, and offering a variety of recreational opportunities. Human activities have, however, imposed stresses on the Great Lakes basins ecological integrity, and one of these stresses the introduction of nonindigenous species of animals and plants is the focus of this report. The opening of the St. Lawrence Seaway in 1959 provided a route into the Great Lakes not only for international maritime trade but also for aquatic invasive species (AIS) carried in the ballast water needed by ships to operate safely. Ships ballast water is not the only vector by which AIS enter the Great Lakes, but it has accounted for 55 to 70 percent of reported AIS introductions since 1959, including that of the zebra mussel (Dreissena polymorpha).