Hordeum is a genus of annual and perennial plants in the grass family. The species are native throughout the temperate regions of Africa, Eurasia, and the Americas.

Species: Species include:

• Hordeum aegiceras – Mongolia, China including Tibet
• Hordeum arizonicum US (CA AZ NV NM), Mexico (Baja California, Sonora, Durango)
• Hordeum bogdanii – from Turkey and European Russia to Mongolia
• Hordeum brachyantherum – Russia (Kuril, Kamchatka), Alaska, Canada including Yukon, US (mostly in the West but also scattered locales in the East), Baja California
• Hordeum brachyatherum – Chile
• Hordeum brevisubulatum – European Russia; temperate and subarctic Asia from Turkey and the Urals to China and Magadan
• Hordeum bulbosum – Mediterranean, Central Asia
• Hordeum californicum – US (CA; OR; NV)
• Hordeum capense – South Africa, Lesotho
• Hordeum chilense – Argentina, Chile (including Juan Fernández Is)
• Hordeum comosum – Argentina, Chile
• Hordeum cordobense – northern Argentina
• Hordeum depressum – US (CA; OR; WA; ID; NV), British Columbia, Baja California
• Hordeum distichon – Iraq
• Hordeum erectifolium – northern Argentina
• Hordeum euclaston – Brazil, Uruguay, Argentina
• Hordeum flexuosum – Uruguay, Argentina
• Hordeum fuegianum – Tierra del Fuego
• Hordeum guatemalense – Guatemala
• Hordeum halophilum – Argentina, Chile, Bolivia, Peru
• Hordeum intercedens (bobtail barley) – California, Baja California
• Hordeum jubatum (foxtail barley) – widespread in US and Canada; Asiatic Russia, Inner Mongolia, Central Asia, Caucasus
• Hordeum × lagunculciforme – Iraq, Turkmenistan, Himalayas, western China
• Hordeum lechleri – Argentina, Chile
• Hordeum marinum (sea barley) – Europe, North Africa, southwestern and central Asia
• Hordeum murinum (wall barley) – Canary Islands, Europe, North Africa, southwestern and central Asia
• Hordeum muticum – Argentina, Chile, Bolivia, Peru
• Hordeum parodii – Argentina
• Hordeum patagonicum – Argentina, Chile
• Hordeum × pavisii – France
• Hordeum procerum – Argentina
• Hordeum pubiflorum – Argentina, Chile
• Hordeum pusillum (little barley) – widespread in Canada and US; northern Mexico, Bermuda, Argentina
• Hordeum roshevitzii – China, Korea, Primorye, Mongolia, Siberia, Kazakhstan
• Hordeum secalinum – Europe, Mediterranean, Caucasus
• Hordeum spontaneum – from Greece and Egypt to central China
• Hordeum stenostachys – Brazil, Argentina, Uruguay, South Africa
• Hordeum tetraploidum – Argentina, Chile
• Hordeum vulgare (barley) – native to the Middle East, now cultivated in many countries

Formerly included species: Botanists now regard many species as better suited to other genera: Arrhenatherum, Crithopsis, Dasypyrum, Elymus, Eremopyrum, Hordelymus, Leymus, Psathyrostachys, and Taeniatherum.

Etymology: The name Hordeum comes from the Latin word for "to bristle" (horreō, horrēre), and is akin to the word "horror".

Distribution: The species are native throughout the temperate regions of Africa, Eurasia, and the Americas.

Ecology: Hordeum species are used as food plants by the larvae of some Lepidoptera species, including the flame, rustic shoulder-knot and setaceous Hebrew character.

Some species have become weeds introduced worldwide by human activities, others have become endangered due to habitat loss.

Uses: One species, H. vulgare (barley), has become of major commercial importance as a cereal grain, used as fodder crop and for malting in the production of beer and whiskey.