London Heathrow Airport, often referred to simply as Heathrow, is the United Kingdom's busiest and best-connected airport. It is the busiest airport in Europe and has the world's highest number of international passenger movements. By total passenger traffic it is the world's third-busiest airport. Heathrow is located in the London Borough of Hillingdon, 15 miles (24 km) west of Charing Cross in Central London.

In 1977, the London Underground was extended to Heathrow, connecting the airport with Central London in just under an hour via the Piccadilly Line.

Terminal 4 was built away from the three older terminals, to the south of the southern runway. The terminal opened in 1986 and became the home for then newly privatised British Airways. In 1987, the British Government privatised the British Airports Authority (now just "BAA plc"), which included seven of Britain's airports, including Heathrow.

Heathrow at present has four passenger terminals (numbered 1 to 4) and a cargo terminal. Permission for a fifth passenger terminal (Terminal 5) was granted in November 2001, and construction is now well under way.

Heathrow is accessible via the nearby M4 motorway (terminals 1–3) and M25 motorway (terminals 4 and 5). There are both drop off and pick up areas at all terminals, and both short and long stay car parks are available. Public transport links are good with connections from three stations on the London Underground Piccadilly Line (Terminals 1-3, Terminal 4 and Hatton Cross), and two on the Heathrow Express line (which is considerably quicker and considerably more expensive; as of February 2003 trains leave every 15 minutes for a 15-minute journey costing £13–£15) directly to London's Paddington station. Heathrow Terminal 4 station is expected to be closed until September 2006 due to Terminal 5 construction work.

Heathrow is the world's third-busiest airport by total passenger traffic, after Atlanta/Hartsfield-Jackson and Chicago/O'Hare in the United States; however, due to the large number of foreign connecting flights, Heathrow actually has the world's highest number of international passenger movements. In 2004 Heathrow was the busiest airport in Europe in terms of total passenger traffic (31.5% more passengers than at Paris Charles de Gaulle Airport or Frankfurt International Airport), but it was only third behind Charles de Gaulle and Frankfurt in terms of plane movements (9.5% fewer planes than at Charles de Gaulle, and 0.3% fewer planes than at Frankfurt), and also only third in terms of cargo traffic (24.8% less cargo than at Charles de Gaulle and 23.2% less than at Frankfurt).

The airport is a regular backdrop for movies. In 2003 it was particularly visible in the Richard Curtis romantic comedy Love Actually. A secret camera installed at the arrivals hall at Terminal 4 captured the reunions between people coming off planes and those meeting them. Snippets of some of the more expressive greetings were played at the beginning and end of the movie. Heathrow is also the set of the BBC/Discovery Wings show Airport. A more surprising appearance of the airport was in the 1964 movie Dr. Strangelove. General Ripper's office at the fictitious Burpelson Air Force Base is decorated with a large aerial photo, presumably of the base; but in fact this is Heathrow Airport, in its old 6-runway configuration. The 2003 BBC Docu-Drama The Day Britain Stopped focused on how a poorly maintained transport infrastructure could cause major disaster, culminating in a major aircraft collision at Heathrow.