This is the first of a series of CD-ROMs on the identification of European limnofaunas, the entire identification system being the result of cooperation between the ETI and several Dutch organisations monitoring water quality. While future CDs in this series will detail the taxonomic groups from family to species level, this one describes and illustrates a total of 314 families, classes or phyla and presents an overview of the invertebrate limnofauna of northwestern Europe. A taxonomic, hierarchical overview and an alphabetical list of all included taxa offer an easy route to information on any particular taxon. A large number of taxa can also be found in an alphabetical list of common names, which are provided in eight European languages. All text in the program is "hyperlinked" to an illustrated glossary offering definitions for over 800 scientific terms. In addition to textual descriptions, over 2000 illustrations (drawings and colour pictures) and 24 movies are presented.