Iran and six world powers are meeting in Vienna this week in their latest attempt to hammer out a comprehensive nuclear agreement by July 20.
That's when a six-month interim agreement expires. It can be extended for up to another six months, though all sides say they're aiming for an agreement this summer.
Iran is negotiating with the so-called P5 plus one, which consists of the five permanent members of the U.N. Security Council — the U.S., Britain, France, Russia and China — plus Germany.
Here are some of the major sticking points as the talks enter a crucial stage:
1.