Small to large terrestrial, lithophytic or epiphytic, rhizomatous herbs; roots elongate, fibrous; rhizome short to elongate. Shoots unbranched, very short to long, erect, leafy, clustered or less frequently well-spaced, glabrous, the lower part enclosed by two to several sterile sheaths, three- to many-leaved above. Leaves one to many, relatively thin-textured, fleshy or coriaceous, plicate or conduplicate, spreading or suberect, ovate, lanceolate, elliptic, ligulate, or oblong, obtuse, acute or acuminate, often tridenticulate at apex, yellow-green, green, or bluish green, plain, chequered or tessellated with dark and lighter green on upper surface or spotted with blackish maroon on upper surface, lighter green below, sometimes finely spotted or flushed with purple at base or all over on lower surface, glabrous or variously pubescent, ciliate or not on the margins. Inflorescence terminal, one- to many-flowered; rachis terete, hairy, glandular or glabrous; bracts conduplicate, linear, elliptic, lanceolate, ovate or oblong, green, sometimes spotted or flushed or striped with purple, glabrous, pubescent or glandular, ciliate or not. Flowers usually showy, concolorous or bicoloured; pedicel obscure to short; ovary unilocular or trilocular, three-ribbed, glabrous or hairy, placentation parietal or axile. Dorsal sepal erect to hooded over lip, ovate, lanceolate, obovate or elliptic, obtuse, acute or acuminate, glabrous or pubescent on the outer surface, sometimes pubescent within at base, ciliate or not, unmarked, spotted or streaked on inner surface. Lateral sepals usually fused to form a concave synsepal that is more or less similar to the dorsal sepal, rarely free to base, apex entire or bifid, sometimes two-keeled on outer surface. Petals free, flat, inflexed or reflexed, spreading or pendent, spiralling or not, elliptic, ovate, lanceolate, linear-lanceolate, linear or oblanceolate, rounded, obtuse, acute or acuminate at apex, unmarked, glabrous or rarely pubescent on outer surface, often pubescent in basal half on inner surface, usually ciliate on margins, sometimes warty on margins, spotted or streaked on inner surface, sometimes veins coloured differently. Lip deeply pouched and inflated, slipper-shaped or urn-shaped, with (or more rarely without) more or less pronounced incurved side lobes which are sometimes reduced to auricles, hairy within especially on lower surface, sometimes spotted within, glabrous or hairy on outer surface; front margin incurved or not, sometimes with short marginal horns; side lobes sometimes warty. Column short, stalked, porrect; anthers two, bilocular, borne on short obtuse to acute filaments; pollen powdery or viscid; staminode terminal on column, sessile or short stalked, often shield-like, rarely reduced or absent, transversely reniform, oblong, ovate, obcordate or linear, flat, convex or longitudinally conduplicate, entire, bifid or tripartite at apex, glabrous to papillose or finely pubescent, ciliate or not; stigma stalked or subsessile, dependent, tripartite, more or less papillose. Capsule erect to pendent, three-ribbed, cylindrical to almost ellipsoidal, sometimes beaked at apex. Seeds spherical to ellipsoidal, with a thin testa except in Selenipedium in which the testa is hard and dark-coloured.