Officials from nearly 200 countries have begun a two-week meeting in Paris to reach a new deal on reducing carbon emissions. The meeting is the 21st Conference of the Parties, and the talks are being dubbed COP21.

Nations have met every year since 1992 to strike a deal on greenhouse-gas emissions. They did that just once—in Kyoto in 1997—and came close to a pact in 2009 in Copenhagen. President Obama, Xi Jinping, the Chinese president, and Narendra Modi, the Indian prime minister, are among the leaders who are attending the UN talks.

Here’s a brief look at what is being discussed and what the potential sticking points are:

What’s being discussed? Negotiators are looking at voluntary efforts to reduce greenhouse-gas emissions past 2020. Each country is setting its own target, and rich countries will pay poorer ones to reduce their emissions—an attempt to offset the perceived costs of developing an economy.

Is an agreement likely? One reason for optimism is the world’s two biggest emitters, China and the U.S., both want an agreement. The two countries were seen as the biggest obstacles to a treaty in Copenhagen in 2009, a failed attempt to reach such a deal.

What are the sticking points this time? One major issue is how much richer countries will pay poorer ones to lower their emissions. Another, perhaps trickier, issue is whether any pact would be legally binding: The European Union wants it to be so; the U.S. doesn’t. India, the world’s third-biggest emitter, has already raised concerns over a potential treaty. And then there is the U.S. Congress, where Republican leaders are already warning that any deal struck in Paris could crumble.

What about the Kyoto Protocol? The 1997 treaty to reduce emissions covered only rich countries. But the pact was never ratified by the U.S. Senate, making it toothless because the United States, which was then the world’s biggest emitter of greenhouse gases, wasn’t bound to the protocol. The agreement also excluded China, now the world’s biggest emitter, and India.