Abstract Fish assemblage indicators developed throughout the world were reviewed and key differences in methodologies depending on ecoregions, basins and contrasting fish fauna summarised. Common elements of existing Indices of Integrity were identified to support the development of a European-wide fish index. These include, using reference condition, accounting for natural fish assemblage variability, evaluating metric precision and selecting the most sensitive and complementary metrics. For future developments, it was recommended to pay more attention to temporal variability in fish assemblages, age structure of key (sentinel) species and fish migration. Testing hypotheses at different steps of the process seems to be the appropriate way to design fish indices.