Perennial, but sometimes short-lived, terrestrial, epiphytic, or lithophytic, autotrophic or rarely mycotrophic herbs (or rarely scrambling vines), with rhizomes, tubers, or rootstocks with mycorrhizal fungi in roots. Stems either sympodial or monopodial, usually leafy, but leaves sometimes reduced to bractlike scales, 1 or more internodes at base often swollen to form a "pseudobulb"; epiphytic species with aerial, photosynthesizing adventitious roots, often bearing 1 or more layers of dead cells (velamen). Leaves 1 to many, alternate or occasionally opposite, often distichous, sometimes terete or canaliculate, glabrous or very rarely hairy, frequently fleshy or leathery, base almost always sheathing, sometimes articulated, sometimes forming a false petiole, margin entire, apex often emarginate. Inflorescence basal, lateral, or terminal, erect to pendulous, racemose, spicate, subumbellate, or paniculate, 1- to many flowered, flowers rarely secund or distichously arranged. Flowers small to large, often quite showy, usually zygomorphic, very rarely ± actinomorphic, bisexual [very rarely monoecious and polymorphic], sessile or pedicellate, most often resupinate with pedicel and ovary twisted through 180°, occasionally not twisted or twisted through 360°. Ovary inferior, 1-locular, placentation parietal (or rarely 3-locular and placentation axile). Sepals usually free but sometimes variously adnate, median (dorsal) one often dissimilar to laterals, laterals sometimes adnate to a column foot to form a saccate, conic, or spurlike mentum. Petals free or rarely partly adnate to sepals, similar to sepals or not, often showy; lip entire, variously lobed or 2- or 3-partite, ornamented or not with calli, ridges, hair cushions, or crests, with or without a basal spur or nectary, margins entire to laciniate. Column short to long, with or without a basal foot, occasionally winged or with lobes or arms at apex or ventrally; anther mostly 1, less often 2 or 3, terminal or ventral on column, caplike or opening by longitudinal slits; pollen usually forming distinct pollinia, less often loose, pollinia 2, 4, 6, or 8, mealy, waxy, or horny, sectile or not, sessile or attached by stalks (caudicles or stipes) to 1 or 2 sticky viscidia; stigma 3-lobed, mid-lobe often modified to form a rostellum, other lobes either sunken on ventral surface of column behind anther or with 2 lobes porrect. Fruit a capsule, rarely berrylike, usually opening laterally by 3 or 6 slits. Seeds very numerous, dustlike, lacking endosperm, rarely winged.

Recent analyses of orchids incorporating data from DNA analyses have confirmed many aspects of the established classifications but have also provided some surprises for orchid taxonomists. First of all, the results have upheld the monophyly (evolutionary integrity, i.e., the group includes all the taxa derived from an ancestral species) of the orchid family, including the apostasioids and cypripedioids. They also suggest strongly that the orchids are an ancient group that evolved in the great southern continent of Gondwanaland before it split up to form the southern continents of Australia, Africa, and South America, the island of Madagascar, and the subcontinent of India. The subfamilies Apostasioideae, Cypripedioideae, and Orchidoideae (sensu Dressler, Phylogeny Classific. Orchid Fam. 1993) are all monophyletic. However, recent work clearly shows that Vanilla and its relatives form a separate and ancient clade (an evolutionary lineage including all the taxa derived from a single ancestral one) that deserves recognition as the subfamily Vanilloideae, that the Spiranthoideae nest within a more broadly defined Orchidoideae, and that Vandoideae are a specialized clade within a more broadly defined Epidendroideae.

A detailed new classification of the orchid family is currently being produced under the title Genera Orchidacearum, of which four of the six volumes have been published and a fifth is near completion (Pridgeon et al., Gen. Orchid. 1-4(1). 1999-2005). Even when this work is completed, such is the speed with which new information and techniques are being developed and published, it will almost certainly require revision. However, we now have the broad bones of a more robust and predictive classification of the family that will be more satisfactory than the presently widely used systems that are based mainly upon morphological characters.

The classification of the family is currently the subject of some debate, particularly the circumscription and the placement of certain tribes, subtribes, and genera. The classification of Chase et al. (in Dixon et al., Orchid Conservation, 69-89. 2003), elaborated in Pridgeon et al. (loc. cit.), which is strongly supported by recent molecular, embryological, and morphological analyses, is followed here. They recognize five subfamilies: Apostasioideae, Cypripedioideae, Vanilloideae, Orchidoideae, and Epidendroideae.

About 800 genera and ca. 25,000 species (some estimates as high as 30,000 species): worldwide, except for Antarctica, most numerous in the humid tropics and subtropics; 194 genera (11 endemic, one introduced) and 1,388 species (491 endemic, one introduced) in five subfamilies in China.