Publisher Summary This chapter describes preventive and corrective methods for improving water quality, both in the watershed and in the waterbody into which it drains. Facilitating the application of preventive methods is the goal. However, corrective methods also can be important if preventive measures already have been applied to the maximum extent. The chapter discusses the methods of water quality management within the watershed in four major categories: pollution prevention from forests and agriculture, clean production by factories, wastewater treatment by municipalities, and ecotechnological methods using absorption of pollution by plant communities in natural and constructed wetlands. The chapter describes seven groups of in-lake measures, which include biomanipulation or eutrophication control based on modifying food chain relations, various methods of lake mixing and oxygenation, hydraulic regulation by modifying the flow through lakes, biomass removal, methods of treating lake sediments, nutrient inactivation methods in the waterbody, methods that treat sediments, and other miscellaneous methods. The chapter illustrates some examples of successfully used combinations of in-lake approaches and discusses some innovative approaches, which show promise.