Future of the eurozone
The eurozone can only work if countries are better off inside the currency union than on the outside. In its current form, the single currency is unsustainable. The success or failure of the eurozone will determine the EU's future, as continued low growth and divergent economic fortunes will further heighten euroscepticism.
Those who believe that an embattled EU, weakened by an ongoing Greek crisis – or worse, Grexit – would give big concessions to David Cameron are misguided.