The genus Daphnia is one of the most widely known groups of the freshwater zooplankton. Member species are used as fundamental models in teaching biology and are widely used as test organisms in toxicology. Despite this, the taxonomy of the group remains confused. The status of the most common taxa has been the subject of debate for most of this century. A considerable effort made over the last twenty years in the molecular genetic analysis of extensive collections made over a range of geographical scales is now bearing fruit for the North American faunas, and is being extended to other regions. The present work has focussed largely on published material in order to synthesise the present state of affairs, and provide a tool to assist the collection of more useful information for the continuing revision of the genus.