Truly aquatic insects species are in North Europe found in ten different orders, representing a wide variety of form and life styles. Most diverse are the dipterans, the two-winged flies and midges, including among others the notorious blood-sucking pests of the boreal and subarctic regions. Other diverse orders are chiefly the beetles and caddisflies, including several hundreds of species occupying most of the available freshwater environments. Besides the aquatic beetles and bugs, insects classified as aquatic normally develop in water, whereas the adult flying forms are terrestrial. Swarming mayflies and hovering libellulas are well-known characteristics of lake and stream shores, whereas their aquatic larvae remain more anonymous for the general naturalist. On the contrary, insect larvae are nowadays widely used in environmental monitoring, the actual species composition at a locality providing reliable evidence on the impacts of acidification and pollution. Environmental applications have engaged many people in the identification of aquatic insect larvae, on scientific, commercial or governmental basis. However, these activities have been hampered by a general lack of an up-to-date standard work providing modern nomenclature and reliable identification keys including all known taxa from the region. The present handbook is designed to fill this gap, as far as taxonomic knowledge is available. Geographically, the handbook focuses on the Nordic Countries, i. e. Denmark, Norway, Sweden and Finland. Most information included will be correct also for the rest of Northern Europe, although some more southern taxa may be absent from the identification keys and check lists. The handbook is designed to serve several important purposes. The introductory text together with the many literature references provided for each order or family is a rich source of information on natural history, morphology, faunistics, classification and identification. The richly illustrated identification keys are generally given for both larvae and adults at least to the generic level. Check lists record the distributions of all known species from the region on a national basis.