To most people, the word weeds signifies pest plants of the farm, roadside, or other upland areas. However, weeds also occur on, under, or near water. Fifteen million dollars have been spent on a single aquatic nuisance in the South—the prolific water-hyacinth which clogs navigable streams and lakes with its growth. Hundreds of thousands of dollars have also been expended on control of the waterchestnut or water caltrop in the Potomac River. In addition, smaller-scale local campaigns are continuing against cattails, reeds, giant cutgrass, maidencane, alligatorweed, needlerush, lotus, rose-mallows, willows, alders, buttonbush, and similar weeds that interfere with land use for recreation, irrigation, or other purposes. Since control of marsh and aquatic plants in wildlife habitat is an unfamiliar activity to most weed workers, broad aspects of its what, why, and how will be explained briefly.