The number of orchid species equals more than twice the number of bird species, and about four times the number of mammal species. It also encompasses about 6–11% of all seed plants. The largest genera are Bulbophyllum (2,000 species), Epidendrum (1,500 species), Dendrobium (1,400 species) and Pleurothallis (1,000 species).

The family also includes Vanilla (the genus of the vanilla plant), Orchis (type genus) and many commonly cultivated plants such as Phalaenopsis and Cattleya. Moreover, since the introduction of tropical species in the 19th century, horticulturists have produced more than 100,000 hybrids and cultivars.

Orchidaceae are cosmopolitan, occurring in almost every habitat apart from glaciers. The world's richest concentration of orchid varieties is found in the Himalayan region of Nepal. The great majority are to be found in the tropics, mostly Asia, South America and Central America, but they are also found above the Arctic Circle, in southern Patagonia, and even two species of Nematoceras on Macquarie Island, close to Antarctica.

The following list gives a rough overview of their distribution:

• tropical Asia: 260 to 300 genera
• tropical America: 250 to 270 genera
• tropical Africa: 230 to 270 genera
• Oceania: 50 to 70 genera
• Europe and temperate Asia: 40 to 60 genera
• North America: 20 to 26 genera