But none of the groups fighting on the ground have been invited.
So absent from the talks will be both the Assad government and the Syrian political opposition, as well as the myriad other rebel militias fighting the Assad regime.
Syria's Kurds won't be present, and of course jihadist militants of the so-called Islamic State (IS) won't be either.
But the hope is that if outside powers can bridge their differences, it could be a first step towards influencing at least some of the warring sides.
And some things have changed since the last abortive diplomatic push to end Syria's war: Iran's nuclear deal has made it a more acceptable partner to the United States and its allies.
Russia has more incentive to push for peace: to keep its military involvement in Syria limited.
The mounting refugee crisis has increased pressure on Syria's neighbours and on European countries too.
And there is growing alarm at the way IS militants keep extending their reach.
The focus (not for the first time) will be on how to agree a political process, including changes to Syria's constitution and new elections, to open the door to a broader transition.
The main sticking point remains President Bashar al-Assad's role, and whether he should be allowed to stay or at some point be forced to stand down.
So where do the most important countries attending the Vienna talks now stand?
The United States and other Western powers (who held a pre-meeting in Paris to present a united front in Vienna) have shifted slightly recently.
They no longer say Mr Assad must go as soon as possible, but rather that a transition must guarantee that he departs under a precise timetable.
Saudi Arabia and Turkey, which had been insistent Mr Assad must go, may also be softening their position somewhat. The Saudi foreign minister on Wednesday still insisted he must step down, but within a specific time frame.
But Russia and Iran, both backing President Assad militarily, claim he is a necessary partner in the fight against IS jihadists, and whether he stays or goes is not a matter for outside powers, but for the Syrian people to decide in elections.
What is less clear is whether behind the scenes they are prepared to be more flexible, or less.
Russia's President Vladimir Putin has repeatedly denounced elected leaders being ousted under pressure. He recently invited Mr Assad to Moscow to show him his support. But senior Russian officials have usually been careful to offer support to Syria's legitimate government, rather than Mr Assad himself.
So would Russia support a compromise?
Iran meanwhile has said there is no reason for Mr Assad to go, since he won re-election last year and his term should last until 2021. Will Tehran make him staying in power a red line?
If there were no agreement in Vienna it would not be surprising, but it could also be dangerous. The Saudi government said it was coming to see if Russia and Iran were serious about a peace process, and if not, it would know not to waste time on them.
Syrian opposition leaders have warned that if this week's diplomacy fails, it could lead to an intensification of fighting on the ground.

Mark Avis and Aaron Saddleton produced the rare feat while playing on the Royal Norwich Golf Club course.
Bookmaker Paddy Power gave the odds of two amateur golfers scoring aces at the same hole as 25 million-to-one.
Morgan Thompson, assistant professional at the club, said the achievement, reported first in the Eastern Daily Press, was "remarkable".
Mr Saddleton, 29, from Norwich, was the first to tee off at the 135-yard hole and was delighted to sink an ace.
But he had little time to celebrate as Mr Avis, 43, of Attleborough, quickly stepped up and repeated the feat.
Mr Thompson said: "I was working in the shop at the time and suddenly saw everybody celebrating.
"I don't think they could quite believe it and it was all they were talking about afterwards."
Mr Thompson said the course averaged one hole-in-one every six months.
The players were on opposing sides in a 2v2 game during a Norfolk Handicapped League match between Sprowston Manor Golf Club and Royal Norwich Golf Club.
These Vienna talks for the first time include all the external countries involved in the Syria conflict Â­including, crucially, Iran.
[NEXT_CONCEPT]
Two golfers from Norfolk have achieved a 25 million-to-one double hole-in-one at the same hole.