Prostrate, erect, or rarely climbing herbs (annual or perennial), subshrubs, shrubs, or rarely small trees, usually with cystoliths (except in following Chinese genera: Acanthus, Blepharis, Nelsonia, Ophiorrhiziphyllon, Staurogyne, and Thunbergia), isophyllous (leaf pairs of equal size at each node) or anisophyllous (leaf pairs of unequal size at each node). Branches decussate, terete to angular in cross-section, nodes often swollen, sometimes spinose with spines derived from reduced leaves, bracts, and/or bracteoles. Stipules absent. Leaves opposite [rarely alternate or whorled]; leaf blade margin entire, sinuate, crenate, dentate, or rarely pinnatifid. Inflorescences terminal or axillary spikes, racemes, panicles, or dense clusters, rarely of solitary flowers; bracts 1 per flower or dichasial cluster, large and brightly colored or minute and green, sometimes becoming spinose; bracteoles present or rarely absent, usually 2 per flower. Flowers sessile or pedicellate, bisexual, zygomorphic to subactinomorphic. Calyx synsepalous (at least basally), usually 4- or 5-lobed, rarely (Thunbergia) reduced to an entire cupular ring or 10-20-lobed. Corolla sympetalous, sometimes resupinate 180º by twisting of corolla tube; tube cylindric or funnelform; limb subactinomorphic (i.e., subequally 5-lobed) or zygomorphic (either 2-lipped with upper lip subentire to 2-lobed and lower lip 3-lobed, or rarely 1-lipped with 3 lobes); lobes ascending or descending cochlear, quincuncial, contorted, or open in bud. Stamens epipetalous, included in or exserted from corolla tube, 2 or 4 and didynamous; filaments distinct, connate in pairs, or monadelphous basally via a sheath (Strobilanthes); anthers with 1 or 2 thecae; thecae parallel to perpendicular, equally inserted to superposed, spherical to linear, base muticous or spurred, usually longitudinally dehiscent; staminodes 0-3, consisting of minute projections or sterile filaments. Disk annular and nectar-producing at base of ovary. Ovary superior, 2-locular, placentation axile, with 2 to many ovules per locule; style filiform, simple; stigma funnel-shaped or 2-lobed (one lobe sometimes suppressed), one or both lobes sometimes recurved or recoiled. Fruit a loculicidal, stipitate or not, 2-valved, explosively dehiscent capsule [rarely drupaceous], 2- to many seeded, apex with or without a beak; septa remaining attached to or separating from inner wall of mature capsule. Seeds usually borne on hooklike retinacula (attached to septa of capsule), or retinacula lacking (Nelsonia, Ophiorrhiziphyllon, Staurogyne, Thunbergia), surface smooth or roughened, pubescent or lacking trichomes, sometimes with hygroscopic trichomes that expand when moistened.

Pollen characters have traditionally been important in delimiting genera and higher taxa in the family; they are used sparingly in the key below.

Avicennia Linnaeus, usually placed in either Verbenaceae (and treated as such in Fl. China 17: 49. 1994) or Avicenniaceae, is now included in Acanthaceae on the basis of molecular phylogenetic studies (e.g., A. E. Schwarzbach & L. A. McDade, Syst. Bot. 27: 84-98. 2002).

The major economic importance of the family is horticultural. Species of Acanthus, Aphelandra R. Brown, Barleria, Crossandra Salisbury, Eranthemum, Fittonia Coemans, Justicia, Odontonema Nees, Pachystachys Nees, Ruellia, Sanchezia Ruiz & Pavon, Thunbergia, and numerous other genera are widely cultivated as ornamentals. Many species are cultivated in China but not treated in the present account. These include: Aphelandra squarrosa Nees, Barleria lupulina Lindley, Crossandra infundibuliformis (Linnaeus) Nees, Eranthemum pulchellum Andrews, Fittonia albivenis (Veitch) Brummitt, Justicia betonica Linnaeus, J. brandegeeana Wasshausen & L. B. Smith, J. carnea Lindley, Pachystachys lutea Nees, Ruellia elegans Poiret, R. simplex C. Wright, Sanchezia parvibracteata Sprague & Hutchinson, Thunbergia erecta (Bentham) T. Anderson, and T. laurifolia Lindley.

Hu Chiachi & Tsui Hongpin. 2002. Acanthaceae. In: Hu Chiachi, ed., Fl. Reipubl. Popularis Sin. 70: 1-309.

About 220 genera and ca. 4,000 species: pantropical and subtropical, with a few species in temperate regions; 35 genera (one endemic) and 304 species (134 endemic, eight introduced) in China.

(Authors: Hu Jiaqi (胡嘉琪 Hu Chia-chi), Deng Yunfei (邓云飞); John R. I. Wood, Thomas F. Daniel)