In this handbook the genetalia of both sexes of all the British Species, with the sole exception of the female H.tigurina Ris, have been refigured, or described for the first time. In cases where very little British material was available, the genetalia were drawn from continental specimens. A brief generic key to larvae has been included, as well as a list of selected references to published larval descriptions. It is hoped that this handbook will stimulate the collection, rearing and identification of the larvae of the British Hydroptilidae so that one of the few remaining gaps in our knowledge of the British Trichoptera may soon be filled.