Once completed, the line will bring up to 31 billion cubic metres of gas a year from the Caspian and the Middle East across Turkey and into Europe.
It will give an important alternative energy supply to Russia, which already meets 30% of Europe's gas needs.
But much still remains to be agreed on, not least where the gas will come from.
The five countries - Turkey, Romania, Bulgaria, Hungary and Austria - have been working on the Nabucco project with the European Commission for seven years now.
But still the decision to sign the heads of government agreement on 13 July has come as a surprise.
To begin with there is still no clear idea as to what has been agreed.
Turkey and the European Commission are still at loggerheads over how much gas Turkey will be able to take from the line, with Ankara claiming that it might be another six months before a final agreement is reached.
More worrying still, Nabucco still has no guaranteed supply of gas.
Iran, Iraq, Kazakhstan, Turkmenistan and Egypt are all considered potential suppliers in the long term.
Currently though, only Azerbaijan is in a position to supply the 15 billion cubic metres a year the line needs if it is to be constructed as planned by 2014.
But two weeks ago, Baku agreed to sell some of that gas to Russia, a move many understood as a warning to the Nabucco partners to sort out their differences or look elsewhere.
In the same way, Monday's signing ceremony is being seen as largely an attempt to persuade Baku that the Nabucco partners can reach an agreement, on some issues at least.
