There have been six summits in the past nine months involving the 28 members of the European Union.
The latest two-day gathering in Brussels is the last for 2015, with Britain's future in the EU on shaky ground as it renegotiates the terms of its membership.
British Prime Minister David Cameron wants an agreement that can reduce immigration, improve business competitiveness in the EU, give the UK more sovereignty, and protect London's banks from the eurozone.
What's being called a "Brexit" – or "British Exit" - is gaining popular support in the UK if Mr Cameron doesn't get the deal.
But David Cameron has declared he will "battle" to get agreement.
"Today is the day that our referendum bill has received royal assent, and so as a result our in-out referendum (will Britain stay in or leave the EU) will be going ahead before the end of 2017,” Mr Cameron said as he arrived for the talks.
“We’re going to have a conversation dedicated to Britain's renegotiation of its position in Europe and I want to see real progress in all of the four areas that I've mentioned.”
“We're not pushing for a deal tonight, but we're pushing for real momentum so that we can get this deal done. So I'm going to be battling hard for Britain."
Mr Cameron is facing stiff resistance on one key front.
The proposal to make immigrants from the other EU states wait four years before claiming "in-work" benefits in Britain has been widely criticised.
Many eastern Europe members claim it contravenes EU laws banning discrimination.
German Chancellor Angela Merkel said she's pushing to avoid a "Brexit", but has also offered the UK a stern warning.
"From the German point of view I will hold the debate in the spirit that we would like to keep Britain in the European Union but at the same time do not want to limit the basic liberties, non-discrimination, free movement, of the European Union," she said.
Similar rhetoric was expressed by French President Francois Hollande.
"If they are in line with European treaties, then sure we can arrive at an end to negotiations. If not the negotiations will be more difficult, because while it is legitimate to listen to the British Prime Minister, it is not acceptable to revise the very basis of European commitments," he said.
Hungarian Prime Minister Viktor Orban believes it might take until next year until an outcome - one way or another - is reached.
"David Cameron's proposals have four major chapters. Three supportable, easy for us, basically we agree. The fourth one is more complicated so we should negotiate more on that. And that's exactly the social issue," Mr Orban told surrounding media.
Britain's demands come as the EU continues to address a superabundance of new arrivals.
One million people have arrived in Europe this year alone, and proposals to boost the zone's external border forces are being considered.
President Hollande said he'll support that proposal, and the bid to resettle Syrian refugees directly from camps in Turkey, thus trying to prevent them taking boats across the Mediterranean.
"External borders must be protected,” he said.
“I absolutely agree with the proposal of the EU Commission to have guards at borders. We must also commit to respecting our word, especially for refugees that need to be moved or relocated but within the limits that were set."
Greek Prime Minister Alexis Tsipras believes a resolution to the crisis cannot be reached without greater solidarity across Europe.
"I think that now is the time for more cooperation in order to decrease migrant flows, and especially non-refugee flows,” Mr Tsipras said.
“We're ready to cooperate more closely with Turkey in order to decrease the flows but our first obligation is our humane obligation to give to these people solidarity."
