Plants trees, sometimes monoecious, unarmed, not producing milky sap. Branches unarmed (occasionally thorny elsewhere). Leaves alternate, petiolate. Stipules small, linear or lanceolate, shed early. Leaf blades simple, pinnately veined or 3-veined from the base, the margins entire or toothed. Inflorescences axillary, small clusters or of solitary flowers, sometimes from the axils of bud scales, thus clustered at the base of new growth, in monoecious species the staminate flowers below the perfect flowers on the same shoot. Flowers perfect or imperfect, actinomorphic (sometimes appearing somewhat zygomorphic at fruiting in Ulmus), hypogynous, stalked or less commonly sessile. Calyces of 4–9 separate or fused sepals, in perfect and pistillate flowers persistent at fruiting. Corollas absent. Stamens 4–9 (absent or highly reduced in pistillate flowers), the filaments more or less straight in the bud, free or fused to the basal portion of the calyx tube, the anthers exserted, attached toward their bases, variously colored, dehiscent by longitudinal slits. Pistil 1 per flower (absent in staminate flowers), of 2 fused carpels, the ovary superior, sometimes short-stalked, 1-locular, with 1 ovule, the placentation apical. Styles 2 or 1 deeply 2-branched, stigmatic along the inner side. Fruits drupes, samaras, or ellispoidal and with a leathery outer layer, 1-seeded. About 16 genera, 165 species, widespread in temperate and tropical regions, mainly in the northern hemisphere.

Recent research involving anatomical features and molecular genetics of the group has established that Ulmus and Planera are not the closest relatives of Celtis. The traditional genera of Ulmaceae are now typically treated in two families. Ulmaceae in the strict sense comprises 7 genera (including Ulmus and Planera) and about 60 total species. However, Celtis and its relatives are now considered members of an expanded Cannabaceae, with 11 genera (including Cannabis and Humulus as well as Celtis) and about 110 total species (Sytsma et al., 2002; Judd et al., 2008). In Missouri, the families are most easily distinguished by the morphology of the leaves (3-veined or palmately compound in Cannabaceae, simple and pinnately veined in Ulmaceae), flowers (sepals free and anthers oriented toward the center of the flower in Cannabaceae, sepals united below and anthers oriented toward the outer margin of the flower in Ulmaceae), and seeds (embryo curved or coiled in Cannabaceae, straight in Ulmaceae). Because the treatment of Cannabaceae that was published in Volume 2 of the present work (Yatskievych, 2006) still treated that family in the traditional sense to exclude Celtis, the account of Ulmaceae below includes the genus Celtis for purely practical reasons.