The pollution of rivers and lakes is of concern because of the threat to public water supplied and because of the damage caused to natural ecosystems. Waterborne diseases remain rife in the Third World, while in developed countries there has been a decline in the quality of water. It has also become apparent that pollutants may destroy ecosystems hundreds of miles from their source. Some of these toxic materials such as lead and PCBs also pose threats to public health. This text describes the sources of various types of pollutants, their effects on aquatic organisms, ecosystems and man, using case histories where appropriate. Ways of mitigating the effects of pollutants by control and ecosystem manipulation are described. Techniques for assessing the effects of pollution, both in field and laboratory, are introduced. The role of the biologist in pollution assessment and control and in the wider sphere of mangement of acquatic resources is also examined.