|Abstract||Good biological indicators to monitor the effects of pollutants on marine organisms must be exceptionally sensitive to environmental change so that a significant reduction in their numbers can be used as a warning of deteriorating conditions before the majority of less sensitive organisms are seriously affected. There are good reasons for focusing on parasites in the search for such indicators. Firstly, there are more parasitic than free-living species and parasitic organisms show enormous biological diversity, reflecting adaptations to the parasitic way of life in different types of host and in diverse sites and environments. Secondly, in metazoan parasites with complex life cycles, the different developmental stages have widely differing biological requirements, so that each stage must be assessed separately, thereby greatly extending the number of potential indicators. Thirdly, many parasites have delicate short-lived free-living transmission stages which are highly sensitive to environmental change. They can therefore be adversely affected by even minor changes in the environment. A reduction in their transmission rate, and consequently in their levels of infection, will serve as an early warning that changes are occurring. Conversely, many ectoparasites are highly resistant to environmental change and will respond to such change by increased levels of infection. As a general rule, infections with endoparasitic helminths with complex indirect life cycles tend to decrease, while infections with ectoparasites with direct single-host life cycles tend to increase, with increasing levels of pollution. However, there are enormous variations in the responses of different parasite taxa to different pollutants.