Herbs perennial or annual, slender, sometimes subshrubs, often root hemiparasites of grasses, ± evergreen. Stems green. Leaves alternate, usually sessile, usually linear or, sometimes, scale-like, 1-3-veined, lateral veins not apparent. Inflorescences apparently terminal, usually racemelike, often paniculate, less often cymose (or axillary and 1-flowered); bracts and bracteoles leaflike, sometimes partly adnate to pedicels; bracteoles 1 or 2 and opposed, rarely 4, sometimes absent. Flowers bisexual, white or yellowish green; perianth tube connate to ovary, limb campanulate, cylindric, funnelform, or tubular, usually deeply (4 or)5-lobed, lobes valvate, usually with a tuft of hair behind each stamen. Stamens (4 or)5, usually on the base of lobes; filaments not exserted; anthers ovoid or ellipsoid, cells parallel, dehiscence longitudinal. Disk superior, inconspicuous or connate to base of perianth tube. Ovary inferior, stalked or sessile; ovules 2 or 3, pendulous from apex of placenta, usually flexuous or corrugate. Style long or short; stigma capitate or inconspicuously 3-lobed. Fruit a small nut, with persistent perianth at apex, exocarp dry, membranous [rarely fleshy], endocarp bony or slightly rigid, usually ridged; embryo in the middle of fleshy endosperm, erect or slightly curved, usually oblique, terete, radicle as long as or slightly longer than cotyledons.

The inflorescence of Thesium has been interpreted in different ways. In the majority of species it is axillary and 1-flowered with a pair of bracts at the apex of the peduncle, which is often fused to the subtending leaf, and with the pedicel proper very short to almost absent. In practice, botanists have often assumed that the inflorescence is a raceme with the peduncles as pedicels and bracts as bracteoles, and for convenience that interpretation has been maintained here.

Thesium chanetii H. Léveillé and T. glabrum Schindler are both Diarthron linifolium Turczaninow (Thymelaeaceae). A record of Thesium australe R. Brown (Prodr. 353. 1810) from Hainan is probably based on a misidentification of the very similar T. chinense Turczaninow.

About 245 species: widely distributed in temperate regions of the Old World, especially numerous in S Africa, a few species in tropical and temperate South America; 16 species (nine endemic) in China.