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. Centrifuges and uranium enrichment
Iran has somewhere in the neighborhood of 20,000 centrifuges capable of enriching uranium for nuclear energy, medical research — or nuclear weapons fuel. Of those, fewer than 10,000 are currently spinning, all relatively inefficient "first generation" models.
Eliminating all of the centrifuges, the "zero-enrichment" option, is considered unrealistic. Washington is hoping, experts say, to limit Iran to a symbolic few-thousand centrifuges.
Iran compares this to an "apartheid" regime for nuclear energy states, with no other signatory to the Nuclear Nonproliferation Treaty, or NPT, facing such restrictions on its enrichment capacity. Experts say defining an enrichment regime that both sides can live with remains the single most difficult issue on the table.
One possible path to a compromise would involve not measuring the number of centrifuges or their efficiency, but maintaining an overall enrichment cap, one that could rise over time as it becomes clear the agreement is holding.
Another variant, laid out by Princeton University researchers, suggests a two-stage approach in which Iran begins with very limited enrichment capacity while being free to develop a greater enrichment capacity for later use as more reactors come online.
2. Arak heavy water reactor
The controversial heavy water facility near the city of Arak could provide an alternative route to nuclear weapons fuel by producing plutonium.
This is precisely how a number of nuclear weapons states got their start. The West initially hoped Iran would offer to scrap the plant, which has been stalled for years. Instead, Iranian officials have suggested that it might agree to modify the reactor so it produces much less plutonium.
One idea has been to get Iran to commit to never building a reprocessing plant for the Arak reactor, without which plutonium cannot be converted to weapons fuel. This, of course, would require aggressive verification measures (see below.)
Some officials say this issue is all but resolved. But under the format of these talks, "nothing is agreed until everything is agreed." In other words, if the most difficult issues, such as enrichment, remain unresolved, the breakthroughs on issues such as Arak will fall as well.
3. Fordow underground enrichment facility
The 2009 announcement that Iran had a secret uranium enrichment plant at Fordow, deep beneath a mountain not far from the holy city of Qom, sent shock waves through the nonproliferation community.
Iran insists it wasn't required to reveal the site's existence to the International Atomic Energy Agency, or IAEA, any earlier, but the bunkered facility fueled suspicions that Tehran was keeping the nuclear weapons option open.
Iran says it will never abandon the Fordow site, or any of its other nuclear facilities. Analyst Ali Vaez has written that one solution would be to convert Fordow into a research and development facility.
4. Possible military dimensions, or PMDs
This has been primarily an issue for the IAEA, which has several questions about past Iranian research and experiments that appear to have been part of weapons research.
Iran insists there are peaceful explanations for all the questions but has been reluctant to provide them, insisting this issue should be dealt with later.
Washington says PMDs, which were also raised in U.N. Security Council resolutions, must be addressed in some fashion in order to reach a nuclear accord. Experts say if Iran were willing to "come clean" about some of its past activities, this issue might be finessed, though the details of such a move remain problematic.
5. Monitoring and verification
This will be a huge component of any nuclear deal. Iran would have to agree to unprecedented levels of intrusive inspections and monitoring of its nuclear program.
Analyst and former U.S. nuclear negotiator Robert Einhorn has written that the IAEA will "require monitoring authorities and procedures not ... found in any existing safeguards agreements," powers that "go well beyond those contained in the (so-called) Additional Protocols," currently the strongest verification regime used by the agency.
For its part, Iran insists that it must be treated like any other nuclear energy state, and is demanding that any additional monitoring be for a limited time only.
6. Sanctions relief
This, in sum, is why Iran's negotiators are at the table. They want to get their country out from under the punitive sanctions crippling the banking and oil and gas sectors, and return Iran to the world economic community.
Under the interim nuclear accord now in effect, sanctions relief has been very limited. The main benefits have been a bump in oil sales and a series of payments of Iran's own money, frozen revenues that were "thawed" by the modest nuclear restrictions Iran has been observing since late January. Because the sanctions relief thus far has been so small, some experts doubt Tehran would be happy about simply extending the interim accord for an additional six months.
One of the trickier aspects of a comprehensive agreement will be the timing and scope of sanctions relief. Iran needs the banking and oil and gas sanctions lifted before its economy can recover, while the West will be reluctant to lift those until it's sure Iran is complying with tougher limits on its nuclear activities.
One additional complication: The comprehensive agreement, if approved, will refer to the lifting of all "nuclear-related" sanctions, but some of the sanctions approved by Congress are not just about nuclear issues; they also refer to things such as Iran's human rights record.
7. Mistrust and suspicion
Although not on the list of nuclear sticking points, overcoming decades of mistrust will be essential if a comprehensive nuclear agreement is to succeed. Western, Israeli and Gulf Arab hardliners believe that no matter what Tehran says, it will never give up its nuclear weapons ambitions.
Iran's hardliners, including Supreme Leader Ayatollah Ali Khamenei, are convinced that Washington will never be satisfied with an entirely peaceful Iranian nuclear program but will press to topple the Islamic Republic.
In response to such entrenched suspicions, supporters of nuclear diplomacy say "consider the alternative." If there is no agreement, they say, there will be two immediate results: The unprecedented international sanctions regime that brought Iran to the negotiating table will begin to crumble, and the most pragmatic and engaging figure in Iranian politics in years, President Hassan Rouhani, will be greatly weakened.