At the time when I last carried out a review of the subject of pesticides and freshwater fauna (Muirhead-Thomson, 1971) it was still possible for a single author to do justice, within one book, to the information then available regarding all forms of freshwater animal life and all types of freshwater body. In the 15 years since that book was published, there has not only been an enormous proliferation of knowledge about this subject but also noteworthy changes in emphasis and priorities. There has been increasing specialisation within this general subject as well, making it increasingly difficult for a single author to encompass all aspects of this problem. For all these reasons, the scope of the present review is restricted to running waters, rivers and streams, and to the macroinvertebrate fauna of such water bodies. The restriction to macroinvertebrate fauna is dictated in part by the fact that a great deal of the voluminous literature in the last 15 years deals with studies on the reactions of freshwater fish, to such an extent that a competent review of that aspect, including all the physiological work on uptake and retention of pesticides by different organs, would require a separate volume. However, one aspect of those fish studies cannot be omitted from any review devoted to aquatic macroinvertebrates, that is the effect of pesticides and allied toxic chemicals on feeding habits of fish in so far as these are influenced by drastic changes in the availability of different invertebrate fish food organisms, as measured by changes in the composition of the stomach contents.