The historic boundary to the north of Berkshire follows the River Thames, from Buscot to Old Windsor. Therefore, the historic county includes territory that is now administered by the Vale of White Horse and South Oxfordshire districts of Oxfordshire, but excludes Slough and Eton, which are historically in Buckinghamshire. Berkshire County Council was the main local government of the county from 1889 to 1998 and was based in Reading, though the County Borough of Reading was administered separately until 1974. In 1974, the county's administrative boundaries were significantly altered. The traditional county town of Abingdon and the areas around Didcot and Wantage were transferred to Oxfordshire, while Slough was added from Buckinghamshire.
Since 1998, Berkshire has been governed by the six unitary authorities of Bracknell Forest, Reading, Slough, West Berkshire, Windsor and Maidenhead and Wokingham. Berkshire borders the counties of Oxfordshire (to the north), Buckinghamshire (to the north-east), Greater London (to the east), Surrey (to the south-east), Wiltshire (to the west) and Hampshire (to the south).
According to Asser's biography of King Alfred, written in 893 AD, its old name Bearrocscir takes its name from a wood of box trees, which was called Bearroc (a Celtic word meaning "hilly"). This wood, perhaps no longer extant, was west of Frilsham, near Abingdon.
Berkshire has been the scene of some notable battles through its history. Alfred the Great's campaign against the Danes included the Battles of Englefield, Ashdown and Reading. Newbury was the site of two English Civil War battles: the First Battle of Newbury (at Wash Common) in 1643 and the Second Battle of Newbury (at Speen) in 1644. The nearby Donnington Castle was reduced to a ruin in the aftermath of the second battle. Another Battle of Reading took place on 9 December 1688. It was the only substantial military action in England during the Glorious Revolution and ended in a decisive victory for forces loyal to William of Orange.
Reading became the new county town in 1867, taking over from Abingdon, which remained in the county. Under the Local Government Act 1888, Berkshire County Council took over functions of the Berkshire Quarter Sessions, covering the administrative county of Berkshire, which excluded the county borough of Reading. Boundary alterations in the early part of the 20th century were minor, with Caversham from Oxfordshire becoming part of the Reading county borough, and cessions in the Oxford area.