This chapter is concerned with the quality of the water environment. Considering that there are well-developed bodies of law at national, European, and international levels, all covering different ground, this is a major topic. The focus here, however, is on the control of pollution of inland and coastal waters; wider issues about water resource management, such as land drainage or flood defence, are not covered in any detail. The various forms of water pollutants are discussed here, as are as their sources and effects. The abstraction of water from the natural environment is only discussed in so far as this has an impact on water quality.