Plants: small to large, in loose to dense mats, light to dark green, sometimes whitish, yellowish, or brownish, pale stramineous to yellow-brown with age. Stems: creeping, ascending, erect, or arching, not reddish, densely to loosely terete-foliate, rarely subcomplanate-foliate or julaceous, irregularly or sometimes regularly pinnate, branches terete-, subcomplanate-, or complanate-foliate; central strand present; pseudoparaphyllia acute to acuminate; axillary hairs of 2–4 cells, cells long, 3–6:1. Stem: leaves appressed, erect, spreading, patent, or falcate-secund, loosely or tightly imbricate, rarely somewhat spaced, lanceolate, ovate, or triangular, slightly to strongly concave, strongly to weakly longitudinally plicate, or plicae irregular, appearing crumpled, or not plicate; base narrowly to broadly short- to long-decurrent; margins serrate, serrulate, or entire, teeth never recurved; apex abruptly or gradually tapered or acuminate, occasionally piliferous; costa to 40–80% leaf length, moderate to somewhat stout, terminal spine present or absent; alar cells usually differentiated, larger or smaller than basal cells, walls thin when cells enlarged, region conspicuous or not, more pellucid or otherwise more opaque; laminal cells usually linear, 8–15:1, occasionally 20:1 or 3–5:1, walls thin to moderately thick, rarely as thick as lumen (B. cirrosum and B. turgidum); basal cells shorter, often wider than more distal cells, walls usually moderately incrassate, region opaque or pellucid across base. Branch: leaves usually smaller and narrower; apex more gradually acute or acuminate. Sexual: condition autoicous or dioicous; perichaetial leaves reflexed, apex gradually or abruptly acuminate. Seta: red-brown with some variation in color in different species, darker with age, rough or smooth. Capsule: inclined, horizontal, or somewhat pendent (erect in B. acuminatum), color similar to that of seta, usually darker, ovate, elongate, or cylindric, straight to curved; annulus not separating or separating by fragments; operculum conic, sometimes broadly short-rostrate; peristome xerocastique, perfect (reduced in B. acuminatum). Calyptra: naked. Spores: 9–25 µm. Nearly worldwide, high Arctic to tropics.
Species ca. 80 (21 in the flora). Many species of Brachythecium exhibit a broad range of variation. Inadequate collections make it difficult if not impossible to separate superficially similar species. Large plants with well-developed stem leaves and fertile collections are most often necessary for the correct identification of Brachythecium. The following key ignores many marginal morphotypes of widespread species, although some of these are discussed in detail in the comments. The present circumscription of Brachythecium is narrower than in previous floras as some species have been placed in Sciuro-hypnum (species related to S. glaciale, S. populeum, S. reflexum, and S. starkei) and Brachytheciastrum (species related to B. collinum and B. velutinum), following M. S. Ignatov and S. Huttunen (2002).