It's being called the mother of all EU summits. Brexit, a possible trade war, enlargement. EU leaders have some heavy lifting to do over the next 48 hours in Brussels. But there's one issue that hangs over all of that, threatening to tear the bloc even further apart.
Migration is firmly on the minds of EU leaders, a real headache for some. So, at the summit, they'll be looking for a medicine they can all agree on.
New migration measures
Tightening external borders, giving Libya more support and creating disembarkation centres outside the bloc for those coming by sea are all on the agenda - along with spending more cash on boosting Frontex border guards.
But some are not convinced that money is the answer.
"I am skeptical about the idea of putting in more money, perhaps it will help, but we already have very low arrival numbers, very low secondary movement numbers and still member states are upholding internal border controls," EU human rights law expert Marie de Somer, told Euronews.
"So, you see that this can not be justified anymore on the basis of sincere public policy considerations. It is more for political reasons, for electoral gains, etc."
What about Schengen?
And all the debates and spats over migration could prove fundamental when it comes to the future of the bloc's crowning achievement - the Schengen passport-free zone.
Marie de Somer commented: "If the problems spill over further than they have already, into Schengen (system), and if we really move towards a system, or a Europe where also the internal border control achievement is lost, then we will have major legitimacy problems I think.
"Also Eurobarometer surveys show that Schengen - the internal free movement zone - is the EU's single largest achievement and it is even valued higher than peace amongst member states."
Make or break?
Euronews' Damon Embling reported from Brussels: "The EU used to be a happy family, now the fault lines are emerging. The big Brexit earthquake, a surge in anti-immigrant and far-right political movements across the continent is evidence of that. It's face is changing.
"EU leaders meeting here in Brussels know they have to act now - at this make or break summit - to secure the bloc's long-term survival."