Canadians are being asked to turn off their lights for an hour tonight as part of a global and symbolic stand against climate change.
Organizers say Earth Hour shows how a small act - turning off lights and electronics for an hour - can have a powerful impact when enough people carry it out.
The World Wildlife Fund, the driving force behind the annual campaign, says millions around the world are expected to take part between 8:30 p.m. and 9:30 p.m. local time.
The organization has said cities have played a key role in promoting the event.
This year, it's also relying on homegrown celebrities to get the word out.
Canadian celebrities including Star Trek's William Shatner, actor and director Jason Priestly and astronaut Chris Hadfield have appeared in short videos meant to get people thinking about conservation.