Today, Saturday 28 March 2009, at 8.30pm, people, businesses and iconic buildings around the world will switch off their lights for an hour - "Earth Hour".
In the UK the initiative is being headed up by the WWF, that is asking people to take part in order to show that they care about people, wildlife and the planet, and that they want the world's leaders to take action to tackle climate change.
More than 2000 cities from 80 countries across the globe have already signed up. In addition, a number of iconic landmarks will be plunged into darkness, including the London Eye, Nelson's Column, the Forth Bridge, the Millennium Stadium in Cardiff, the Eiffel Tower, Niagara Falls, Christ the Redeemer in Rio de Janeiro, Table Mountain in Cape Town and Sydney Opera House.
There's more information on the WWF's site as well as Earth Hour communities on Flickr, YouTube, Facebook and Twitter.