The county of Northumberland included Newcastle upon Tyne until 1400, when the city became a county of itself. Northumberland expanded greatly in the Tudor period, annexing Berwick-upon-Tweed in 1482, Tynedale in 1495, Tynemouth in 1536, Redesdale around 1542 and Hexhamshire in 1572. Islandshire, Bedlingtonshire and Norhamshire were incorporated into Northumberland in 1844. Tynemouth and other settlements in North Tyneside were transferred to Tyne and Wear in 1974 under the Local Government Act 1972.
Lying on the Anglo-Scottish border, Northumberland has been the site of a number of battles. The county is noted for its undeveloped landscape of high moorland, now largely protected as the Northumberland National Park. Northumberland is the least densely populated county in England, with only 62 people per square kilometre.
Northumberland originally meant 'the land of the people living north of the River Humber'. The present county is the core of that former land, and has long been a frontier zone between England and Scotland. During Roman occupation of Britain, most of the present county lay north of Hadrian's Wall. It was controlled by Rome only for the brief period of its extension of power north of the Antonine Wall. The Roman road Dere Street crosses the county from Corbridge over high moorland west of the Cheviot Hills into present Scotland to Trimontium (Melrose). As evidence of its border position through medieval times, Northumberland has more castles than any other county in England, including those of Alnwick, Bamburgh, Dunstanburgh, Newcastle and Warkworth.
Northumberland has a rich prehistory with many instances of rock art, hillforts such as Yeavering Bell, and stone circles such as the Goatstones and Duddo Five Stones. Most of the area was occupied by the Brythonic-Celtic Votadini people, with another large tribe, the Brigantes to the south.
Later, the region of present-day Northumberland formed the core of the Anglian kingdom of Bernicia (from ca. 547), which united with Deira (south of the River Tees) to form the kingdom of Northumbria in the 7th century. The historical boundaries of Northumbria under King Edwin (reigned 616–633) stretched from the Humber in the south to the Forth in the north. Following the battle of Nechtansmere its influence north of the Tweed began to decline as the Picts gradually reclaimed the land previously invaded by the Saxon kingdom. In 1018 its northern part, the region between the Tweed and the Forth (including Lothian that contains present-day Edinburgh), was ceded to the Kingdom of Scotland.