Studies in the UK and elsewhere have shown that acid freshwater habitats have different floras and faunas and fewer species in most taxonomic groups, when compared with near-neutral waters. The presence or absence of certain common species that are sensitive to acid waters can be used to assess the prevailing chemistry of a water body. The method described in this Field Book enables managers to do this for forest streams in upland areas using a limited number of readily recognised invertebrate animals. It may also be used for streams outside woodland. The method is suitable for qualitative monitoring over a number of years to show trends in the ecological status of selected streams. This publication is still available in hard copy.