Administrative (pop., 2001: 479,028), geographic, and historic county, southern England.
It is bordered by the River Ouse valley in the north; its county seat is at {{link=Aylesbury">Aylesbury . It was affected by each phase of English settlement, from the Neolithic to the Saxon. Under Saxon rule, as part of the kingdom of Mercia , it resisted Danish invasion and became prosperous. Before the 20th century it was a rural area, but the modern growth of London and the expansion of transportation links with it have brought population growth.