Perennial, rarely annual, terrestrial, epiphytic or rock-dwelling herbs, arising from creeping-rhizome, annually renewed tuber or thickened rootstocks or rhizome; aerial stem may be swollen at base into pseudo-bulb, some with adventitious or aerial roots. Leaves parallel-veined or curviveined, entire, mostly basal, base of leaf sheathing or not; usually reduced to bracts or scales in saprophytic species. Flowers bisexual, zygomorphic, very rarely regular, in spikes, racemes or panicles, or solitary; perianth mostly petaloid, 6 segments in 2 whorls, free or variously joined, occasionally fused to form tube; if axis erect, labellum on upper side in young bud, in most cases pedicel twisting during development so that labellum is below column; sepals 3, often differentiated into dorsal sepal and 2 lateral sepals; petals 3, differentiated into 2 laterals petals and a labellum. Labellum modified greatly (in all genera but Thelymitra) in size and shape, often with outgrowths such as calli, hairs and fringes; nectary sometimes contained within spur or sac from labellum, or more rarely, the dorsal sepal. Column bearing both anther and stigma, a more or less erect, usually fleshy, prolongation of floral axis; anthers 1 or 2, 2-locular, turned inward, opening by longitudinal slits; pollen grains rarely free, usually aggregated in tetrads or in larger masses that are mealy, waxy or quite hard, one end of which may be extended into sterile part (caudicle); stigma borne on labellum-side of column, 2–3-lobed, median lobe sometimes replaced by sterile outgrowth (rostellum), part of which may be modified into a viscid disc (viscidium); ovary inferior, 1-celled with 3 parietal placentae, rarely 3-celled. Fruit a capsule, usually dehiscing by 3 or 6 longitudinal slits; seeds numerous and very minute, often drawn out at each end or rarely winged, without endosperm, embryo not differentiated.
World-wide distribution with over 850 genera and 25,000–35,000 species; over 190 genera and over 1,300 species in Australia, and 32 genera with over 300 species in Victoria.