WWF's Earth Hour, the annual event which sees many cities turning off non-essential lights for an hour, takes place on Saturday 29 March.
The event began in Sydney, Australia, in 2007, and has spread to over 150 countries and 7,000 towns and cities worldwide.
As well as households and small businesses, large London landmarks will take part, including The Houses of Parliament, Buckingham Palace, Tower Bridge, The Barbican, The Gherkin and the London Eye. You can see the full list of participating landmarks here, as well as some great before and after images of venues with their lights off. We got a bit click-happy with that one.
Lights will be switched off at 8.30pm local time to create a Mexican wave of darkness across the world. We'd love to see it from space. The event begins in Samoa, a full 14 hours ahead of London, and ends in the Cook Islands, 10 hours after London dims the lights.
It's expected that 10 million Brits will take part this year — sign up here if you'll be taking part, and check out the map showing people who have signed up all over the world. Can we make London the most Earth Hour friendly city in the UK?
Previous Earth Hours have generated enough publicity and funding to support several environmental projects around the world, including the replanting of half a million trees in Uganda's Earth Hour forest.
Watch the video for key parts of last year's Earth Hour around the world — including the lights going out on the Houses of Parliament.
Share your ideas for thing to do in London when the lights go out (keep it clean) below.
Houses of Parliament image by Mike Murphy on flickr.