A lot of progress has been made already in talks that have been going on for 19 months. But the sticking points that remain are significant.
How can the security of the Turkish Cypriots be guaranteed if Turkey's estimated 30,000 troops leave?
Should some stay or should Turkey retain the right to intervene?
If not, who or what could act as a guarantor - the UK? Or the EU, of which of course Cyprus is already a member?
How much more territory should Greek Cypriots gain to reflect the fact that they make up the majority of the island's population?
Where should the lines on the maps be drawn?
What should happen to the properties that Greek Cypriots had to abandon in 1974?
Should they get the right to take their old homes back or be compensated and if so by how much?
How should the two sides share power?
There is talk of a rotating presidency but how would that work and could a Turkish Cypriot president really represent the country from time to time at EU summits?
And then there is the really hard question - what kind of a deal would be acceptable to the peoples of Cyprus?