A review is given of the critical events in nomenclature which have affected the systematics of the genus Leptochilus during the past 113 years. Three genera which were previously considered to be synonymous with Leptochilus are now considered valid. These are Microdynerus Thomson, Parodynerus Saussure, and Paralionotulus Bluethgen. Odynerus exilis Saussure is designated as the type-species of the genus Parodynerus.

Published biological information is summarized and new information added. Biological habits of 16 species are discussed. Evolutionary trends in the genus are discussed and the relative specializations of the species groups are compared and diagrammed. A brief account of the world distribution of Leptochilus and a key to the North American genera of eumenids are included.

The 53 species comprising the North American fauna of Leptochilus are relegated to 13 species groups which are characterized and keyed. Of the species recognized in this paper, 37 are newly described; new taxa are: anthracinus, aridulus, autumnus, boharti, brachialis, cacaloa, californicus, chiricahua, crocatus, dolius, ellenae, elongatus, ferrugineus, gibberus, humerus, irwini, isthmus, krombeini, labrosus, lemniscatus, marshi, menkei, mesolobus, milleri, mixtecus, monticolus, oculatus, pachuca, paiute, periallis, petilus, quirogae, sonorae, stangei, tropicanus, washo, and zacatecus. The following 4 names are newly synonymized: Ancistrocerus belizensis Cameron = L. zendalus (Saussure), Odynerus rufobasilaris Ashmead = L. rufinodus (Cresson), Odynerus monotylus Bohart, and Odynerus tylocephalus Bohart = L. acolhuus (Saussure).