The signing of the Lisbon Treaty is designed to bring an end to years of haggling, first over the failed European constitution, and then over the drafting of a new text.
But the debate will go on, at least until the treaty is ratified by the 27 member states - or rejected as the constitution was. Here prominent figures in European politics share their views of the new draft.
Click on the links below to read what they have to say.
Labour MP Gisela Stuart helped draw up the original European Constitution as a member of the convention chaired by ex-French president Valery Giscard d'Estaing
The Lisbon Treaty is to all intents and purposes the old EU constitution under a different name.
If you lay the new text side by side with the old version that was rejected by French and Dutch voters in 2005 you can see that 96% of it is a word-for-word carbon copy.
I'd like to think I should know better than most - after all I sat on the body that helped draw up the original version in 2004.
This is a deeply dishonest process.
As the chief author of the constitution, Valery Giscard d'Estaing, says: "All the earlier proposals will be in the new text, but will be hidden and disguised in some way."
Our government claims that the new treaty is different because we have defended key "red lines".
But these are almost identical to the "red lines" that Tony Blair inserted into the text in 2004 - a text he felt important enough to warrant a national vote.
The Lisbon Treaty would abolish the national veto - our right to say No - in 61 areas, over issues such as energy, health and foreign policy.
It would allow EU judges to decide for how long we should lock up our most dangerous criminals and what rights should be granted to migrants
And it would tighten further the EU's guidelines on public spending, which discriminate against long-term investment in services like schools and hospitals
Whether or not you support the treaty it is clear that it would be a big change in the way we are governed
That's why the people deserve to be asked first.
Slovenian European Affairs minister Janez Lenarcic will have the task of persuading other member states to ratify the treaty, during Slovenia's stint as EU president in 2008.
Slovenia supports the EU Reform Treaty because we believe that it will improve the efficiency of the EU and enhance its democratic legitimacy.
The institutions and their decision-making procedures that were originally designed for a Union of six or 15 Members need to be adapted so as to enable the enlarged EU to function effectively when dealing with the new challenges in the age of globalisation.
Moreover, the new treaty would strengthen the role of the national parliaments, as well as that of the European Parliament, in the daily functioning of the EU.
This would increase transparency and should bring the EU closer to its citizens. In addition, the treaty contains a number of other provisions aimed at improving the EU performance in many policy areas.
In our view, all member states should ratify the treaty, for several reasons:
The EU needs such a treaty in order to function better
The treaty is good for Member States - their views and special concerns were fully taken into account in the negotiation process
All governments that sign the treaty are expected to do everything in their power in order to secure its ratification and entry into force. After all, their signatures are understood to be on behalf of their states and citizens
For the sake of the European project, we must not fail again. Another failure - after the failed ratifications of the now defunct constitutional treaty in two Member States in 2005 - would cause a serious political crisis. This must be avoided if we care about the fate of the European integration project.
The country holding the rotating EU presidency obviously cannot ratify the treaty for others. It can only do it for itself, as each member state is solely responsible for its own ratification process and its outcome.
Nevertheless, as Slovenia will assume the Presidency at a critical time - only a couple of weeks after the signing ceremony in Lisbon - our early ratification could serve as an example.
That is why our government intends to initiate the ratification procedure in the Slovenian Parliament immediately after the signature of the treaty.
The presidency will also make itself available for any assistance that individual member states may seek in the process of ratification.
Pierre Moscovici, a French Socialist MP, was France's Europe minister from 1997 to 2002, and, like Gisela Stuart, a member of the convention that helped draft the constitution.
European Union leaders reached agreement in October on an outline of new rules to govern the 27-member bloc, shaking the EU out of its lethargy.
Although the text is typified by reticence and caution, signalling a clear reduction of European enthusiasm, key elements of the European constitutional treaty have been salvaged.
The institutional innovations laid down in the constitution were mostly preserved, in order to improve the EU's decision-making capacity.
Limited innovations, such as an "exit clause" and references to climate change, were also introduced.
The new treaty will not mark a political watershed for the EU.
Rather, it is the tool box, the mechanism for decision-making that the Europe of 27 needs.
Romano Prodi (Italian Prime Minister) has claimed that the European Union has lost its common spirit to move ahead.
And some rejoice in this "mini-treaty" because they want a "mini-Europe", while militants for a political Europe regret the decline of a certain ambition.
But even if the Lisbon Treaty doesn't change the face of the EU, it is the indispensable condition for relaunching the European project.
Of course the text will not be sufficient on its own.
It will solve Europe's institutional crisis, but not its political and moral crisis.
For that, the EU needs leaders for whom Europe is a mission, and not merely a constraint.
Harry van Bommel is Europe affairs spokesman for the Dutch socialists.
The Socialist Party (SP) is the biggest opposition party in the Dutch parliament and was the successful leader of the No campaign in 2005.
Although the Labour Party promised in the general election that a referendum would be held on any new constitutional treaty, the government in which they participate decided not to organise a referendum on the European Reform Treaty.
It says that the new treaty does not have constitutional implications. It would represent merely a routine series of amendments, and in terms of style, content and magnitude would be completely different from the constitutional treaty.
The British think-tank, Open Europe, calculated that 96% of the proposals contained in the constitutional treaty recur in the new treaty.
Various European leaders have also admitted that the core of the constitutional treaty is preserved.
In both treaties important veto rights will be surrendered.
In addition, a permanent president and a minister of foreign affairs will be instituted, even if the latter will be known instead as the High Representative.
These proposals imply the further transfer of sovereignty to the European level, without any satisfactory democratic control being exercised over this.
In our view, it's up to the people to decide whether to transfer sovereignty to the EU.
Therefore a new referendum is needed.
Although a majority in the Dutch parliament has already said No to a new referendum, we persist in putting forward a proposal for a referendum and call a vote on it before the Reform Treaty is ratified.
Roland Rudd, Chairman of Business for New Europe, a lobby group supporting the UK's active engagement in a free-market EU.
The treaty is a logical response to the expansion of the European Union designed to make an EU of 27 countries work more effectively.
We should remember that Britain was and is in the forefront of supporting enlargement, which has transformed the countries of Eastern and Central Europe and brought enormous benefits to established member states and the EU as a whole.
These enlargements have fundamentally altered the shape, nature and direction of the European Union.
The reform of the EU's institutions, such as ending the rotating presidency and a fairer voting system in the European Council, are very positive steps.
The UK will have a greater share of the vote in the Council.
As with previous treaties, the Reform Treaty has moved some areas to majority voting (QMV), and in this case such moves are aligned with UK policy priorities such as energy and development.
Even Margaret Thatcher, who could never be described as an arch Europhile signed the Single European Act, which involved moves to QMV.
The business community is sanguine about these reforms.
It understands that the structural changes are necessary in a growing organisation and that cross-border challenges such as climate change are most usefully addressed in international fora like the EU.
Talk of a superstate is hyperbolic nonsense.
Despite the so-called EU monster, the Queen still sits on the throne, we still set our own taxes and interest rates and retain the ability to declare war and follow our own foreign policy.
The important thing about the EU is that there is a great opportunity, with the mood and current crop of European leaders, for the treaty to be complemented by a programme of reform.
It must not be missed.