Cornwall is a unitary authority and ceremonial county of England, within the United Kingdom. Cornwall is a peninsula bordered to the north and west by the Celtic Sea, to the south by the English Channel, and to the east by the county of Devon, over the River Tamar. Cornwall has a population of 536,000 and covers an area of 3,563 km² (1,376 sq mi).

The administrative centre, and only city in Cornwall, is Truro, although the town of St Austell has the largest population.

Cornwall forms the westernmost part of the south-west peninsula of the island of Great Britain, and a large part of the Cornubian batholith is within Cornwall. This area was first inhabited in the Palaeolithic and Mesolithic periods. It continued to be occupied by Neolithic and then Bronze Age peoples, and later (in the Iron Age) by Brythons with distinctive cultural relations to neighbouring Wales and Brittany.

There is little evidence that Roman rule was effective west of Exeter and few Roman remains have been found. Cornwall was the home of a division of the Dumnonii tribe whose tribal centre was in the modern county of Devon, then known as the Cornovii, separated from the Brythons of Wales after the Battle of Deorham, often coming into conflict with the expanding English kingdom of Wessex before King Athelstan in AD 936 set the boundary between English and Cornish at the Tamar. From the early Middle Ages, British language and culture was apparently shared by Brythons trading across both sides of the Channel, evidenced by the corresponding high medieval Breton kingdoms of Domnonee and Cornouaille and the Celtic Christianity common to both territories.

Historically tin mining was important in the Cornish economy, originating in the 5th century BC and becoming increasingly significant during the High Middle Ages and expanding greatly during the 19th century when rich copper mines were also in production. In the mid-19th century, however, the tin and copper mines entered a period of decline. Subsequently china clay extraction became more important and metal mining had virtually ended by the 1990s. Traditionally fishing (particularly of pilchards), and agriculture (particularly of dairy products and vegetables), were the other important sectors of the economy. The railways led to the growth of tourism during the 20th century, however, Cornwall's economy struggled after the decline of the mining and fishing industries. The area is noted for its wild moorland landscapes, its long and varied coastline, its many place-names derived from the Cornish language, and its very mild climate. Extensive stretches of Cornwall's coastline, and Bodmin Moor, are protected as an Area of Outstanding Natural Beauty.

Cornwall is the traditional homeland of the Cornish people and is recognised as one of the Celtic nations, retaining a distinct cultural identity that reflects its history. Some people question the present constitutional status of Cornwall, and a nationalist movement seeks greater autonomy within the United Kingdom in the form of a devolved legislative assembly, and greater recognition of the Cornish people as a national minority.