There has been intense research and wide public interest in hormonally active micropollutants present in the aquatic environment for more than a decade. This group of micropollutants contains a large number of structurally diverse chemicals with quite different origins. Some are natural hormones, others pharmaceuticals, both veterinary and human, and yet others are chemicals which are hormonally active by accident, not design. Most of these hormonally active micropollutants enter the aquatic environment in effluent from (STWs). Hence, the highest concentrations of these chemicals will be in rivers heavily impacted by effluent, especially effluent from poorly performing STWs. In some cases river concentrations of a hormonally active micropollutant are well established, but in many cases they are not. It is important to know "real world" concentrations, so that laboratory ecotoxicological studies can be conducted at environmentally relevant concentrations.Fish, as vertebrates, have endocrine systems very similar to those of mammals. This means that if a micropollutant is hormonally active in a mammal, it is very likely to also be active and cause similar or related effects, in fish. It is currently less clear whether or not these chemically are also hormonally active in invertebrates, whose endocrine systems often differ appreciably from those of vertebrates.