The genus includes annual or short-lived herbaceous perennial plants, and shrub-like perennials, sub-shrubs or small shrubs.
Leaf shape is diverse across the genus. They are simple in some commonly cultivated species; in others they are pinnately toothed, or pinnate, sometimes multiple pinnate and dissected. In most species the leaves are covered in fine hairs or indumentum, which normally contain the essential oils.
Flowers are borne in whorls, held on spikes rising above the foliage, the spikes being branched in some species. Some species produce coloured bracts at the apices. The flowers may be blue, violet or lilac in the wild species, occasionally blackish purple or yellowish. The calyx is tubular. The corolla is also tubular, usually with five lobes (the upper lip often cleft, and the lower lip has two clefts