London is a huge travel hub, and when you are flying overseas, there is a good chance you will have to transfer there. Whether you are visiting Europe, Africa or Asia, many flights connect in London (especially when flying from the USA). But unlike transferring in other large cities, where you just have to find your way around the airport between flights, you might have to change airports in London.
There are six airports in London, though Heathrow and Gatwick are the most popular for international flights. So if you are transferring in London, make sure you know which airport you need to be at for your connection. You might have to get all the way across the city, rather than just across a terminal.

