Administrative (pop., 2001: 479,028), geographic, and historic county, southern England. It is bordered by the River Thames, London, and the River Ouse valley in the north; its county seat is at 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.