Annual to perennial herbs, rarely (not in Australia) undershrubs, glabrous to densely minutely scabrous. Leaves opposite; stipules absent; bracts herbaceous. Flowers solitary, terminal or often in few-flowered dichasia, often few-flowered cymes, usually 5-merous, bisexual; sepals usually 5, free; petals entire or slightly emarginate, white; stamens (8–)10; ovary 1-celled, with many ovules; styles 3, rarely 4 or 5. Capsule ampulliform to more or less conical, opening by twice as many teeth as styles; seeds numerous, discoid or slightly nautiloid, tuberculate.
About 250 species indigenous to the Northern Hemisphere; 2 species naturalised in Australia.