A taxonomic revision, including descriptions, diagnoses, and bibliographies, is given of the genera and species of the family Nepticulidae (Lepidoptera) so far collected in Canada. In addition, extensive material from the U.S.A. and a number of specimens from Europe have been examined. Five new species and two new subspecies are described. Notes on the biology of the species have been collated from rearing records, label data, and the relevant literature. With one exception all existing types have been examined and, in addition, a number of types designated.A major feature of this work is a study of the genitalia of the moths — never before undertaken extensively on the North American Nepticulidae. These structures give valuable diagnostic features and are of fundamental importance in the provision of modern descriptions and keys. As genitalia characters are thought to be the best taxonomic indicators of relationships between species in the Nepticulidae, the species-groups proposed have been mainly defined on these structures.The confusion over the nomenclature of the family name is discussed. Taxonomic histories of the genera are given and one generic synonymy is made.Taxonomic problems are discussed in relation to distribution and food-plant choice. The host-plant associations of the family are reviewed, with reference to the mining habits of the larvae. A priori use of host-plant data in the taxonomy of the Nepticulidae is found to be limited. However, the proposed species-groups illustrate a general conclusion that related species tend to feed on plants that are themselves taxonomically related.