Is there enough time to negotiate a trade deal?
The aim is to have a deal done in time for the end of 2020. That is a very challenging timetable.
Trade negotiations tend to take several years to complete. They are technically challenging and politically contentious. Both those features can make them drag on.
To take one example, the EU's deal with Canada took more than five years for negotiators to complete and another three before it came into force, on a provisional basis.
The UK-EU negotiation will be unusual in that it is intended to establish a trade relationship that is less integrated than what the two sides have now.
Usually, trade negotiations make for closer commercial relations, so past experience isn't necessarily a good guide to the likely timetable.
Some people say that because we are already fully aligned with the EU the negotiation will be easy and quick.
But for many Brexit supporters the freedom to depart from EU rules is one of the main prizes. How much we depart - on food standards for example - will be important for the EU in judging what restrictions to impose on British goods. That could be a time-consuming process.