EU leaders have signed a new treaty in Rome which sets up the first European constitution and expands the EU's powers and authority. BBC News Online examines the issues.
What is the significance of this ceremony?
The EU constitution was agreed in Brussels on 18 June. But it still has to be ratified by all 25 member states.
At least nine countries plan to hold referendums on it - and a referendum is still possible in Germany too and a handful of other countries. There is growing public scepticism about the European project - not only in Britain.
Blair's vision of Europe contrasts with the Franco-German vision
The choice of Rome is highly symbolic. The 1957 Treaty of Rome - signed by France, Germany, Italy, Belgium, the Netherlands and Luxembourg - is one of the EU's foundation stones. It established the European Economic Community.
The constitution is designed to make the EU run smoothly after 10 new members joined the union on 1 May. It will make the EU more than simply a free trade area, but less than a United States of Europe.
The document sees a big extension in the number of policy areas where countries will lose their national veto. There is to be a full-time president overseeing the co-operation between member states, along with a foreign minister.
This is the culmination of years of hard bargaining over the future powers of the EU. It represents a compromise between those who are pushing for deeper integration and those who want to protect national sovereignty.
When does the constitution come into effect?
Ratification by all 25 EU members is expected to be a protracted affair, with countries setting their own timetables. But they must ratify it within two years - before Bulgaria and Romania join as planned in 2007.
It will become law only if ratified in every member state. If any country rejects it in a referendum, the way forward is unclear.
The country in question could hold a second vote - possibly after negotiating opt-outs - or it could leave the EU, or the constitution could be altered or abandoned.
Referendums have given the EU problems in the past. Irish and Danish voters have rejected key EU treaties, then approved them in a re-vote.
Meanwhile, there are fears that the enlarged EU's institutions will function inefficiently without a new constitution.
Will national sovereignty be undermined?
The constitution preserves some nation-state rights, but it confirms that the states have given up certain rights - in the areas of the internal market, foreign trade, agriculture, fisheries and the environment, for example.
February 2002: Convention starts work
June 2003: Draft submitted to EU Thessaloniki summit
December 2003: Brussels summit fails to agree final text
May 2004: EU enlarges to 25
June 2004: Text agreed
The constitution sets the goal of a common foreign policy, but the process of reaching such a policy is complex and each member state has the right to opt out.
The constitution creates an EU foreign minister post, but states must agree foreign policy unanimously.
In the key areas of defence and taxation, too, member states can go their own way.
But there will be more joint action to be decided by majority voting - in immigration and asylum policy, for example.
The "red lines" the UK secured during constitution negotiations, for example, will allow it to decide policy in areas such as taxation, defence, foreign policy and immigration.
Aren't EU institutions in disarray now?
The date for the new EU Commission to start work has been set back by the row with the European Parliament over some commissioners-designate, especially Italy's Rocco Buttiglione.
The incoming EU Commission President, Jose Manuel Barroso, has to come up with a new team that will satisfy MEPs - as soon as possible. The new Commission was originally due to be in place on 1 November.
The debacle has overshadowed Friday's ceremony in Rome - and it is especially embarrassing for the host, Italian Prime Minister Silvio Berlusconi, who put forward Mr Buttiglione.
Mr Buttiglione, a staunch Roman Catholic put forward for the post of justice and home affairs commissioner, recently remarked that homosexuality was a "sin" and that single mothers were "not very good people" - comments which outraged many MEPs.
EU leaders - who nominate the commissioners - are expected to hold intensive discussions on Friday in an effort to help Mr Barroso get his team right.
It is unclear how voters across Europe will view the row in the long term. Eurosceptics argue that it shows EU institutions are too cumbersome and that MEPs are challenging the national right to nominate commissioners.
But many MEPs say the affair has demonstrated their power - they talk of the parliament having "come of age", of "democracy in action".