Plants with tuberoids or rhizomes, terrestrial or epiphytic, occasionally saprophytic. Leaves radical or cauline, sometimes reduced to sheaths. Inflorescence one to many-flowered. Flowers zygomorphic, hermaphrodite, usually reversed (resupinated) by torsion of ovary or pedicel. Perianth-segments in 2 whorls: the outer of 3 ± equal segments (sepals), the inner with 2 lateral equal (petals) and one unequal, medium (labellum). Labellum highly modified in shape and coloration, with or without a spur. Style and stamen ± fused to a column (gynostemium). Fertile anthers 1 or 2, the steriles transformed into staminodes near base of column. Fertile stigmas 2-3, usually confluent, a sterile modified into the rostellum, a tissue separating the anther from the fertile stigma. Pollinia 2, sometimes 2-4 parted, or 4, waxy or powdery-granulose, often attached by stalk-like caudicles to 1 or 2 viscid glands (viscidia), which can be free or enclosed in 2 or a single pouch (bursicula). Ovary inferior, sessile or pedicellate; placentation parietal, 1-locular, rarely 3-locular. Fruit a capsule, longitudinally dehiscing by 3 or 6 slits. Seeds tiny, numerous, without endosperm.