The North American Aegeriidae are revised on the basis of the study of late-instar larvae of about 60 species. These species represent 20 genera in five tribes in two subfamilies: Bembeciini, Zenodoxini, Aegeriini and Melittiini in Zenodoxinae and Synanthedontini in Synanthedontinae. The presently recognized genera, Ramosia Engelhardt, Conopia Hübner and Thamnosphecia Spuler are reduced to synonyms of Synanthedon Hübner. Three new genera are defined by larval characters but are referred to only as Genera I, II and III. Larval characters for most of the species are discussed and illustrated; keys to species, genera and tribes are provided. Tribes, genera and most species are arranged in the text as nearly as possible according to their phylogenetic relationships as suggested by the larvae.