Perennial or annual herbs with radical or alternate leaves, rarely climbing shrubs with opposite leaves. Stipules absent, or adnate to the petiole. Flowers regular or irregular. Sepals 5 or more, rarely fewer, deciduous, often petaloid, imbricate or valvate in bud. Petals often undeveloped, or 5 or more, sometimes only 3, frequently minute or deformed. Stamens many, hypogynous; anthers adnate, dehiscing laterally. Carpels usually numerous, seldom only 1, usually free, 1-celled; style simple; seeds 1 or more on the ventral suture, anatropous, erect with a ventral, or pendulous with a dorsal raphe. Fruit of 1-seeded achenes or many-seeded follicles. A large order dispersed all over the world, not rare in the tropics. Many species are acrid, and some highly poisonous, especially the Aconites.