Northumberland is a traditional, ceremonial and administrative county in northern England. The ceremonial county borders Cumbria to the west, County Durham to the south and Tyne and Wear to the south east, as well as having a border with the Scottish Borders council area to the north, and nearly eighty miles of North Sea coastline. As the kingdom of Northumbria under King Edwin, the area's historical boundaries stretched from the Humber in the south to the Forth in the north. The traditional county covers a smaller area, similar to the modern ceremonial county but also including Newcastle-upon-Tyne, the traditional county town. Being on the border of Scotland and England, Northumberland has been the site of many battles.

Once part of the Roman Empire and the scene of many wars between England and Scotland, Northumberland has a long and complicated history. This explains the many castles in Northumberland, including among the better-known those at Bamburgh, Dunstanburgh, Warkworth and Alnwick. The region of present-day Northumberland once formed the core of the Anglian kingdom of Northumbria. Northumberland is called the "cradle of Christianity" in England, because it was on Lindisfarne, a tidal island north of Bamburgh, also called Holy Island, that Christianity flourished when monks from Iona were sent to convert the English. Lindisfarne was the home of the Lindisfarne Gospels and Saint Cuthbert, who is buried at Durham Cathedral. Northumberland has a history of revolt and rebellion against the government, as seen in the Rising of the North in Tudor times.

Northumberland played a vital role in the industrial revolution. The region's coalfields fuelled industrial expansion in other areas of the country, and the need to transport the coal from the collieries to the Tyne led to the development of the first railways. Ship-building and armaments manufacture were other important industries. Today, Northumberland is still largely rural. Like most English shire counties Northumberland has a two-tier system of local government. Northumberland has traditions not found elsewhere in England, reflecting a mix of indigenous, Celtic, Norse and Anglian influences. The links between Northumberland and Scotland are audible in the dialects of both, which include many Old English words, such as bairn for child.

Geordie refers to a person from the Tyneside region of England and the adjacent former coal mining areas of southeastern Northumberland or the "dialect" spoken by these people. Although the dialect, and some would say language, of Northumbria is called Geordie, the term Geordie is often incorrectly used to cover all the peoples of the North East of England, though this usage is generally confined to people from other parts of the United Kingdom, and can be considered an insult by North-Easterners who do not come from Tyneside, due to intense local rivalries. To North-Easterners the term exclusively refers to persons from Tyneside. Geordie derives much less influence from French and Latin than does Standard English, being substantially Angle and Viking in origin. The accent and pronunciation, as in Lowland Scots, reflect old Anglo-Saxon pronunciations, accents and usages.

The physical geography of Northumberland is diverse. It is low and flat near the North Sea coast and increasingly mountainous toward the northwest. The Cheviot Hills, in the northwest of the county, consist mainly of resistant Devonian granite and andesite lava. A second area of igneous rock underlies Whin Sill (on which Hadrian's Wall runs), an intrusion of carboniferous Dolerite. Both ridges support a rather bare moorland landscape. Either side of Whin Sill the county lies on carboniferous limestone, giving some areas of karst landscape. Lying off the coast of Northumberland are the Farne Islands, another Dolerite outcrop, famous for their bird life. There are coal fields in the southeast corner of the county, extending along the coastal region north of the river Tyne. The term sea coal likely originated from chunks of coal, found washed up on beaches, that wave action had broken from coastal outcroppings.

Approximately a quarter of the county is protected as the Northumberland National Park, an area of outstanding landscape that has largely been protected from development and agriculture. The park stretches south from the Scottish border and includes Hadrian's Wall.