The SE (South Eastern) postcode area covers a broad radial swathe of the south-east of the London post town from the Albert Embankment to West Heath and the nearest edges of Sidcup and Selhurst. It loosely corresponds to the boroughs of Lewisham and Greenwich plus indicated parts of the boroughs of Croydon (north), Lambeth (east), Bexley (west) and Bromley (its northwest corner).

SE1 has a very long tidal Thames frontage and is in Central London as is SE11 named after Kennington, close to Westminster. SE2-SE18 spread across the north and east of the postcode area; enquiring into their naming system explains how SE2, SE7, SE8, SE10, SE16 and SE18 also front the river. SE28 is in the northeast corner of the first group. The postcode area maps roughly to the combined area of the London Borough of Southwark, London Borough of Lewisham and Royal Borough of Greenwich. the eastern part of the London Borough of Lambeth, the northern part of the London Borough of Croydon, the western part of the London Borough of Bexley and the northwestern part of the London Borough of Bromley are within the postcode area.

In 2002, some residents in West Heath, a slight projection with neighbouring areas, wished to have their postcodes changed from SE2 to the adjacent DA7 postcode district, citing higher insurance for their houses, belongings and cars as reasons to change. Royal Mail has said it will not consider changes to postcodes for these reasons.