Identification of larvae of aquatic Diptera (true flies) is complicated by a range of factors, including reliance on examination of mouthparts at the very beginning of many keys, as well as an unclear distinction between aquatic and terrestrial habits for many species. Even at family level, these can cause problems. This review briefly introduces the history of keys to Diptera larvae, with particular reference to Europe, and identifies use of characteristics that may cause problems for the non-specialist, along with attempts made to mitigate these problems. It considers the validity of using identification features that do not require examination of mouthparts, giving examples of those used in previous keys. It then reviews Diptera families that are not normally considered to have aquatic representatives, concluding that in Europe six new families need to be added to the list of freshwater fauna: Cecidomyiidae (rivers and tree holes), Scatopsidae (tree holes), Anisopodidae (tree holes), Bibionidae (probably wetlands), Pachyneuridae (saturated dead wood) and Lonchopteridae (river edges). Elsewhere in the world, a further nine families have been recorded from fresh waters, all from submerged dead wood or from water-filled plant structures such as pitcher plants and tree holes. A dichotomous key to European families of aquatic Diptera is included, based purely on external morphology without reference to mouthparts, and incorporating the six families highlighted above. This key is copiously illustrated with line drawings, mainly of whole animals.