Perennial, terrestrial, epiphytic or lithophytic herbs or rarely scrambling climbers, sometimes saprophytic. Mycorrhizal fungi in the roots and often elsewhere. Growth either monopodial or more commonly sym-podial with rhizomes, tubers, or rootstocks. Roots adventitious, often aerial, sometimes assimilatory, with velamen tissue in epiphytic and some terrestrial species. Stems usually leafy, the one or more inter-nodes often swollen in sympodial species to form pseudobulbs. Leaves glabrous or occasionally hairy; usually entire, rarely palmate or lobed; alternate or occasionally opposite, often distichous, with plicate or convolute vernation; membranaceous to coriaceous, often terete or canaliculate, reduced to scale-like bracts in some species; almost always with a basal sheath, sometimes articulated at the base of the lamina or with a false petiole. Inflorescences erect to pendent; spicate, racemose or paniculate; one- to many-flowered; basal, lateral or terminal, the flowers rarely secund, distichous or subumbellate. Flowers small to large, often showy; zygomorphic but appearing almost actinomorphic in some cases; bisexual or rarely unisexual and polymorphic; sessile or variously pedicellate, most often twisted through 180°, occasionally not twisted or twisted through 360°. Sepals three, usually free but sometimes variously connate, the dorsal sepal often dissimilar to the lateral sepals, the lateral sepals sometimes adnate to the column foot to form a saccate, conical or spur-like mentum. Petals three, free or rarely adnate in part to the sepals, the medial petal distinguished from the others as the lip or labellum. Lip entire, variously lobed or two- or three-partite, often ornamented with calli, ridges, hair cushions or crests, with or without a basal spur or nectary, margins entire to laciniate. Column short to long, formed from stylar and filamentous tissue, with or without a basal foot, occasionally winged or with lobes or arms apically or ventrally; fertile anthers one (rarely two or three), terminal or incumbent, cap-like or dehiscing by longitudinal slits; pollen shed as monads or tetrads, often agglutinated into discrete masses called pollinia; pollinia mealy, waxy or horny, soft to hard, sectile or not, 2, 4, 6 or 8, sessile or attached by caudicles or stipites (tegula or hamulus) to one or two sticky viscidia forming a pollinarium; stigma 3-lobed, the midlobe often modified to form a rostellum, the other lobes either sunken on the ventral surface of the column behind the anther or with two lobes porrect. Ovary inferior, unilocular with parietal placentation or rarely trilocular with axile placentation. Fruit a capsule, usually opening laterally by three or six slits, rarely baccate; seeds numerous, dust-like, rarely with hard seed coats, lacking endosperm, sometimes markedly winged.