Thousands of world leaders and campaigners have gathered in Copenhagen in Denmark to try and agree on the best way to fight climate change.
Some people are calling this conference the most important gathering ever for the future of our planet.
Over the next two weeks, more than 15,000 people will take part in talks on how we can cut pollution.
Experts say countries need to reduce the amount of the gas carbon dioxide they produce to fight global warming.
The conference is being organised by a group of countries called the United Nations (UN).
Loads of scientists think that carbon dioxide and other gasses - called greenhouse gasses - produced by things like our cars, power stations and factories are heating up our planet.
They reckon that's causing big problems with our weather systems, setting off flooding in some parts of the world, and droughts in others.
They also think it's melting the polar ice caps and threatening the habitats of many animals.
One of the UN's top climate experts, Yvo de Boer, told the BBC that many countries are now making promises to reduce the amount of greenhouse gases they produce.
During the meeting countries are hoping to agree a deal that not only cuts the amount of pollution produced, but also provides money for poorer countries to cope with the effects of global warming.