Annual herbs, often spiny; stems stout, often much-branched. Leaves petiolate, alternate, simple or lobed. Capitula unisexual, sessile, in terminal and axillary clusters, males above, females below. Male capitula subglobose; involucral bracts free, few, 1–3-seriate; receptacle hemispherical, with scales; florets many, tubular; corolla 5-lobed; anthers obtuse at base, with a short apical appendage. Female capitula ovoid; involucral bracts 2-seriate, outer few free, inner 2 connate and completely enclosing florets, hard, apically 2-beaked, covered with hooked spines; florets 2; corolla absent; style with filiform branches protruding through holes below beaks of involucre. Cypselas obovoid or obloid, compressed and one larger than the other (in Victoria), retained inside burr-like involucre which is shed as a unit; pappus absent.
About 30 species (or as few as 3 species if a more conservative taxonomy is followed), widespread in tropical and temperate regions of the world; 8 species naturalised in Australia.