Border reivers were raiders along the Anglo-Scottish border. They included both English and Scottish people, and they raided the entire border country without regard to their victims' nationality. They operated in a culture of legalised raiding and feuding. Their heyday was in the last hundred years of their existence, during the time of the House of Stuart in the Kingdom of Scotland and the House of Tudor in the Kingdom of England.

The lawlessness of the Anglo-Scottish Borderlands in the 16th century is captured in a 1542 description of Tynedale and Redesdale:

[Inhabitants there]...nothinge regard[ed] eyther the lawes of God or of the kinges majesties for any love or other lawful consideracion, but onely for the drede and feare of instante coreccion.

Amidst centuries of lawlessness, poverty and low-intensity conflict, compounded by significant invasions along the Anglo-Scottish frontier, familial groups gradually coalesced into organised units of common defence and offence based on kinship, giving rise to what would later be formalised as the 'Surnames.' The reivers emerged between 1350 and 1450, with their activities reaching their height in the 16th century during the Tudor period in England and the late Stewart period in Scotland. Reiving was a matter of subsistence for the borderer. They were infamous for raiding, blood feuds, eliciting protection money or taking hostages (blackmail), where justice was frequently negotiated through arbitration at Truce Days rather than enforced and mandated by state law. Many crimes, such as theft and feuding, were treated with less severity due to the ancient customs and culture of the Borderlands, which had evolved over centuries to tolerate and codify such practices in the March law.

Although less well-known than Highlanders in Scotland, whom they met and defeated in battle on occasion, the border reivers played a significant role in shaping Anglo-Scottish relations. Their activities were a major factor in ongoing tensions between the two kingdoms, and their raids often had international repercussions. There is debate over how great their threat and the extent to which their raids were state-directed rather than purely opportunistic.

The culture of the border reivers, characterised by honour, close family bonds and self-defence, has been said to influence the culture of the Upland South in the United States. Many Borderers migrated as families to America, where their values are thought to have contributed significantly to the region's social structure and political ideologies, with echoes of their influence persisting even today.

Reive, a noun meaning raid, comes from the Middle English (Scots) reifen. The verb reave meaning "plunder, rob", a closely related word, comes from the Middle English reven. There also exists a Northumbrian and Scots verb reifen. All three derive from Old English rēafian which means "to rob, plunder, pillage". Variants of these words were used in the Borders in the later Middle Ages.

The earliest use of the combined term "border reiver" appears to be by Sir Walter Scott in his anthology Minstrelsy of the Scottish Border. George Ridpath (1716?–1772), the author of posthumously published The Border-History of England and Scotland, deduced from the earliest times to the union of the two crowns (London, 1776), referred not to 'border reivers' but only to banditti.