This is the third in a series of manuals designed to facilitate the recognition of the Geometridae of Canada. In the introductory section key characters are given for the identification of Geometridae in adult, larval, and pupal stages. A generalized description of each stage is followed by a key to the subfamilies of the Geometridae and a synopsis of the characters of the Ennominae. Keys to the five parts of the Ennominae (sometimes broken down into tribes) as they are to be treated in this series of manuals are supplemented by text descriptions, line drawings, and colour photographs. The distribution of many of the 45 species is shown on 29 maps. These maps and the studies of the writer suggest the potential value of collecting along some of the major Canadian waterways, such as the Saskatchewan River. This along with collecting in the short-grass prairie and its stream beds in Alberta and Saskatchewan and faunal investigations in the northern end of the Appalachian Mountains in Quebec and New Brunswick should add much to our distribution records.On the basis of adult, larval, and pupal characters a phylogenetic chart of the boarmid group has been constructed.