Plants perennial, sometimes with rhizomes, forming tufts, small to large clumps, or colonies. Flowering stems erect to less commonly ascending or arched, glabrous. Leaf sheaths glabrous or the lowermost roughened or hairy, sometimes glaucous, the ligule short, membranous, rarely absent. Leaf blades flat or with the margins inrolled, glabrous or roughened to hairy, sometimes with a pair of small auricles at the base. Inflorescences erect or arched to nodding, usually with the base held above the uppermost leaf sheath (except sometimes in E. virginicus), with numerous erect to ascending or spreading spikelets spaced densely or loosely along the axis, the internodes 3–25 mm long in various species. Spikelets single or in clusters of 2(3 or 4) at each node of the inflorescence, all similar in size and appearance, all fertile, linear in outline, with 2–8 florets, disarticulating above the glumes or sometimes also at the spikelet base (disarticulating at the nodes of the inflorescence axis in E. longifolius, the joints shed as a unit with the attached spikelets). Glumes present and similar in size and appearance, less commonly highly reduced and uneven or absent, elliptic‑lanceolate to linear, sometimes thickened and hard at the base, the tip with a long or short awn (except sometimes in E. virginicus), glabrous or roughened to hairy. Lemmas with the tip pointed or awned, rounded on the back, glabrous or roughened to hairy, lacking stiff, spinelike hairs along the keel and the margins. Paleas rounded to bluntly pointed at the tip (except in E. canadensis, with pointed to often cleft tips), as long as or shorter than the body of the lemmas. Fruits 4–6 mm long, linear‑oblong, usually somewhat flattened and grooved, dark brown to reddish brown. About 300 species, nearly worldwide.

The generic taxonomy of Elymus is confusing and controversial. Many of the species with spikelets occurring singly along the axis were placed in Agropyron in the older literature (Steyermark, 1963), but that genus has become restricted to a small group of closely related Old World species (see treatment above). The genera Hystrix Moench and Sitanion Raf. were formerly recognized as distinct, based on minor innovations of spikelet morphology and placement, but generally are now accepted as sections within Elymus (even in the strict sense). On the other hand, the relatively broad circumscription of Elymus accepted here includes several large species groups that have been segregated into such genera as Elytrigia Desv., Leymus Hochst., Pascopyrum Á. Löve, Pseudoregneria (Nevski) Á. Löve, and Thinopyrum Á. Löve, among others. These genera generally are based on cytogenetic analysis of genome types and breeding systems, and some are more easily circumscribed morphologically than others. A number of authors (Estes and Tyrl, 1982; Arnow, 1987) have expressed doubts as to the utility of breaking Elymus apart into these more narrowly defined groups. Recently, Assadi and Runemark (1995) studied chromosomal homology in artificial hybrids of various Old World taxa and concluded that although Agropyron (as treated in the present work) has a genome that is well differentiated from that of Elymus, other groups, including Elytrigia, Pseudoregneria, and Thinopyrum, should continue to be treated as part of a broadly circumscribed Elymus.

Hybridization between Elymus species is relatively common, as well as hybridization between Elymus and related genera, such as Hordeum. However, few such hybrids from Missouri are represented in herbaria. Collectors who find plants with unusual morphology should note other species of Triticeae that may be growing in the vicinity.