Dytiscid beetles comprise a large element within the Canadian fauna yet their identification is difficult for the nonspecialist and the distribution and habitats of the species are only sketchily known. The initial and continued aims are to make the predaceous water beetle fauna of Canada accessible to as wide an audience as possible by summarizing available knowledge of the fauna, documenting the species present and their distribution and habitats, providing means for the identification of the taxa, and indicating where knowledge is especially lacking and where interesting and possibly productive research avenues lie. To achieve these aims it has been necessary to revise several genera, review the systematic position of many taxa and to seek out immature stages of various taxa. Much new information, including descriptions of new taxa, is included in this book. The aim of the work being to aid and encourage others to look at water beetles with increased interest and appreciation and to stimulate research on these poorly understood insects. The Canadian fauna cannot be understood without reference to the overall Nearctic fauna and therefore the scope gradually enlarged over time to include most of the North American fauna. This work treats all Nearctic genera and most species. Detailed treatment is provided for all species known from Canada. Non-Canadian species are treated as fully in some genera, in others these species are included only in keys. Only within genera where some species are too poorly known to allow their definition, are all species not included. The areas with the most incomplete coverage are both the Gulf States and southwestern United States, namely western Texas to southern California. For the rest of the continent, it is hoped that this work will provide a useful identification guide.