Clann Eanruig (pronounced KLAHN YAHN-reegk) is the Gàidhlig (Scots Gaelic) name for the Scottish clan known as "the Hendersons" in English. The words "Scot," "Scots" (not scotch), "Scottish," and "Scotland" derive from the Latin word "Scotus" meaning a Celtic inhabitant of Hibernia (Ireland) at the time of the Roman occupation of southern Britannia (Great Britain), i.e., an Irishman. In the Third Century AD, the Scots of Dál Riata began migrating from northern Ireland to the northern part of Great Britain which the Romans called Caledonia. The immigrant Scots settled among another Celtic people known to the Romans as the Picts or Caledoni. The Picts tolerated the newcomers, and even intermarried with them. In 563 AD, Columcille (Saint Columba), an Irish nobleman turned missionary, established a monastery on the Isle of Iona to bring Christianity to both the Scots and Picts.

The Scots of Caledonia flourished and soon outnumbered their Pictish neighbors. Pictish rulers sought to protect their ancient ways from the vibrant new culture from Ireland. Nechtan mac Derile became King of Picts in 706. King Nechtan embraced the new Roman Christian Church (which paid homage to the Bishop of Rome and worshiped on Sunday) to block the power of the Celtic Christian Church (the faith of Columcille which remained independent and continued to worship on the Sabbath.) King Nechtan retired to a monastery in 724. In 843, Kenneth mac Alpin united the Picts and Scots in a common kingdom. Gàidhlig soon became the dominate language, and eventually Pictish culture was absorbed into a common Scottish identity.

The ancient Picts and Scots followed the Celtic custom of matrilineality. This meant that sons could not depend on their father's status, but instead had to establish domains of the own. Pictish prince Eanruig Mor mac Righ Nechtan (Big Henry the son of King Nechtan) established a distinguished family line. The descendants of Prince Henry were known collectively as “clann Eanruig” meaning the “family of Henry.” The males of the clan took the surname “mac Eanruig” meaning “son of Henry,” which was later translated into English variously as “Henryson,” “Henderson,” “McHenry,” “McHendry,” “MacKendrick,” and such. The females of the clan took the surname “nic Eanruig” meaning “daughter of Henry.” A woman normally kept her own clan surname after marriage, and she could usually depend on her clan's support in a dispute with her husband. Families could give children the clan surname of either their mother or father. Over time, the descendants of other prominent Henrys also took the family name "clann Eanruig." Eventually, the most prominent of these families coalesced into a single clan identity.

In contrast to the social equality of the Celtic Picts and Scots, the Germanic Angles and Saxons were patriarchal and patrilineal societies. The father ruled the family and all family members took his surname. After England conquered Scotland, the Scots gradually adopted many of these customs.

Though a small clan, the Hendersons rose to prominence in Caithness, Glencoe, the Shetland Islands, Liddlesdale, and Fordell. In Caithness, Clan Henderson associated with Clan Gunn. In Glencoe, Clan Henderson forged a close alliance with the powerful Clan Donald. In 1692, King William III, suspecting the loyalty of Clan Donald, secretly set the Campbells upon the MacDonalds and Hendersons in the Massacre of Glencoe. After the Massacre, many Henderson families emigrated to Ulster and North America. During the Highland Clearances from 1746 to 1822, many more Henderson familes left Scotland for Ireland, England, North America, Australia, New Zealand, and other lands.

Hendersons in the British colonies of North America played important roles in the drive for American independence from Britain. Patrick Henry of Virginia urged armed revolution with his cry "I know not what course others may take, but as for me, give me liberty or give me death!" Ulster-born physician James McHenry served as George Washington's Secretary of War. (The Star-Spangled Banner, the national anthem of the United States of America, depicts the British naval bombardment of Fort McHenry near Baltimore in 1814.) Hendersons loyal to the British Crown played important roles in the British settlement of Canada, Australia, and New Zealand. Today, Hendersons encircle the globe.

The surnames of the Clan Henderson include d'Handresson, Eanrig, Eanruig, Enderson, Endherson, Endirsone, Enrick, Enrig, Henders, Henderson, Hendersone, Hendersonne, Hendersoun, Hendersoune, Hendery, Hendirsone, Hendirsoun, Hendirsoune, Hendrie, Hendriesoun, Hendrisone, Hendrisoune, Hendron, Hendry, Hendryson, Hendrysone, Henerson, Henersoun, Hennerson, Hennersoune, Hennryson, Henresoun, Henreysoun, Henrici, Henricus, Henrie, Henriesone, Henriesoun, Henrison, Henrisone, Henrisoun, Henrisoune, Henrisson, Henry, Henryesson, Henryson, Henrysone, Henrysonne, Henrysoun, Henrysoune, Inrick, Inrig, Kendrick, Kenrick, MacCanrig, MacCanrik, MacEanruig, MacEnrick, MacHendric, MacHendrick, MacHendrie, MacHendry, MacHenrie, MacHenrik, MacHenry, MacKanrig, MacKendree, MacKendric, MacKendrich, MacKendrick, MacKendrie, MacKendrig, MacKendry, MacKenrick, MakAnry, MakCanryk, MakHenry, McCandrie, McCanrig, McCanrik, McHendry, McHenrie, McHenrik, McHenry, McKanrig, McKendree, McKendrick, McKendry, McKinriche, M'Canrie, M'Cenrik, M'Henri, M'Henry, M'Inrig, M'Kendrig, NicEanruig, and other variants.

The surname spelling variations arose from regional pronunciation differences, and sometimes perversely creative spelling. Some individuals used multiple surname spellings, and sometimes different surname forms. For example, a traveling Henderson might use the surname MacEanruig in the Scottish Highlands, Henderson in the Lowlands, McHenry in Ulster, and Henry in England.

The prefixes "Mac", "Mak", "MC", "MC", "Mc", and "M'" are equivalent, and are all pronounced "mahk". The letter following the prefix may be either capital or lower case. Some Scottish families dropped a "Mac" prefix from their surnames during the Highland Clearances in an effort to curry favor with the Crown. After King George IV visited Scotland in 1822, some of these families resumed using a "Mac" prefix. (The resurgent popularity of "all things Scottish" even induced some Lowland and English families to add an incongruous "Mac" prefix to their surnames.)

The Henderson surnames are sometimes confused with similar sounding surnames of other families. The Hendricks, Hendrix, Hendrickse, Hendricksen, and Hendrickson surnames originated in the Netherlands. The Henricks and Henrik surnames originated in Norway.