The Thames Valley region is a loose term for the English counties or parts of those counties roughly following the course of the River Thames as it flows from Oxfordshire in the west to London in the east. It is generally taken to include parts of Oxfordshire, Buckinghamshire, Berkshire, Surrey and sometimes two boroughs of southwest London. It rarely includes either the source of the Thames at Thames Head in Gloucestershire, or any part of Wiltshire, and does not include the informal entities Central London or the Thames Gateway which surrounds the Thames Estuary.