The River Brent is a river in west and northwest London, England, and a tributary of the River Thames. 17.9 miles (28.8 km) in length, it rises in the Borough of Barnet and flows in a generally south-west direction before joining the Tideway stretch of the Thames at Brentford.

A letter from the Bishop of London in 705 suggesting a meeting at Breġuntford, now Brentford, is the earliest record of this place and probably therefore that of the river, suggesting that the name may be related to the Celtic *brigant- meaning "high" or "elevated", perhaps linked to the goddess Brigantia.

(For the purposes of this section, the Brent catchment area is taken to include the catchment areas of all its tributaries.)

The catchment area varies in altitude from just over 150 metres above sea level at Bushey Heath, on its northern watershed, to barely 10 metres at the confluence of the Brent with the Thames at Brentford.

Broadly speaking, the catchment area can be divided into three topographical zones:

- a southern zone, lying south of a line from about Harlesden to Northolt, which is of low relief and which (apart from the hill at Hanger Lane) lies below an altitude of 40 metres;

- a basin-like north-western zone which is surrounded by areas of higher ground which rise fairly steeply to the west (Harrow on the Hill), north (Bushy Heath) and east (Mill Hill), and where several isolated hills such as Horsenden Hill, Barn Hill and Dollis Hill are located; and

- a north-eastern zone consisting of the relatively narrow and steep-sided valleys of the Dollis Brook, Folly Brook and Mutton Brook; and of the high ground which surrounds those valleys (at Hampstead Heath, Whetstone, Totteridge, Chipping Barnet, etc).