This book first reviews the biology of Dytiscidae or water beetles, including life history and ecology. It then defines and keys adults & larvae (when known) of dytiscid fauna of Canada, the United States, and for some taxa also northern Mexico. The focus is on the fauna of Canada & Alaska, and adults of the 276 species known from this region are treated in detail. For each Canadian-Alaskan species, the following information is presented: nomenclature & synonymy; selected references; description, including illustrations of taxonomically important characteristics; comments on classification or variation; notes on ecology; and description of the species range, accompanied by a map of collection records. Checklists of the dytiscid fauna of Canada/Alaska are also presented, with the distribution of the species recorded by province/territory and Canadian ecozone. Includes systematic index.