Annual or perennial herbs, less often shrubs or trees. Leaves opposite or whorled, rarely alternate, simple, usually entire; stipules minute or absent. Inflorescence a racemose spike, cymose panicle or a solitary, axillary flower. Flowers actinomorphic or rarely zygomorphic, bisexual, perigynous, sometimes heterostylous; hypanthium prominent, sometimes spurred; tooth-like appendages sometimes present between sepals; sepals usually 4–6, valvate, inserted on a floral tube; petals usually 4–6, free, crumpled in bud, rarely absent; stamens usually in 2 whorls and twice the number of petals, sometimes in 1 whorl and equal in number to, or fewer than the petals; anthers versatile or basifixed, longitudinally dehiscent; ovary superior, 2–6-celled; stigma often capitate. Fruit usually a capsule, dehiscing by various means; seeds usually numerous, with little or no endosperm.
28 genera with about 660 species, widespread throughout tropical regions of the world, less common in temperate regions; 8 genera and c. 24 species in Australia.