The Lisbon Treaty became law on 1 December 2009, eight years after European leaders launched a process to make the EU "more democratic, more transparent and more efficient".
Under EU rules, the treaty had to be ratified by all 27 member states before coming into force. The last country to ratify the treaty was the Czech Republic, which completed the process on 3 November 2009.
Like the proposed European constitution before it, the treaty is often described as an attempt to streamline EU institutions to make the enlarged bloc of 27 states function better. But its opponents see it as part of a federalist agenda that threatens national sovereignty.
The planned constitution was thrown out by French and Dutch voters in 2005. The Lisbon Treaty which succeeded it was rejected by Irish voters in June 2008. But it got overwhelming support in a second referendum in the Irish Republic on 2 October 2009.
How similar is Lisbon to the draft constitution?
It contains many of the changes the constitution attempted to introduce, for example:
- A politician chosen to be president of the European Council for two-and-a-half years. But ministerial meetings will still be chaired by the country holding the six-month rotating EU presidency.
- A new post - called High Representative - combining the jobs of the existing foreign affairs supremo, Javier Solana, and the external affairs commissioner, Benita Ferrero-Waldner, to give the EU more influence on the world stage.
- The European Commission will continue to have 27 commissioners - one from each member state. The previous Nice Treaty envisaged a smaller commission - and that idea was to be kept, but it was then dropped as a concession to the Irish Republic in 2008.
- A redistribution of voting weights between the member states, phased in between 2014 and 2017 - qualified majority voting based on a "double majority" of 55% of member states, accounting for 65% of the EU's population.
- New powers for the European Commission, European Parliament and European Court of Justice, for example in the field of justice and home affairs.
- The parliament will be on an equal footing with the Council - the grouping of member states' governments - for most legislation, including the budget and agriculture. This is called "co-decision".
- Removal of national vetoes in a number of areas, including fighting climate change, energy security and emergency aid. Unanimity will still be required in the areas of tax, foreign policy, defence and social security.
Most European leaders acknowledge that the treaty preserves the main substance of the constitution.
If it contains the same substance, why is the Lisbon Treaty not a constitution?
The constitution attempted to replace all earlier EU treaties and start afresh, whereas the new treaty amends the Treaty on the European Union (Maastricht) and the Treaty Establishing the European Community (Rome).
It also drops all reference to the symbols of the EU - the flag, the anthem and the motto - though these will continue to exist.
How long did it take to agree the treaty?
A declaration issued at the EU's Laeken summit in 2001 called for a Convention on the future of Europe to look into the simplification and reorganisation of the EU treaties, and raised the question whether the end result should be a constitution.
The Convention began work in February 2002 and a constitution was signed in Rome two-and-a-half years later, in October 2004. But that text became obsolete when it was rejected by French and Dutch voters in 2005.
Work began in earnest on a replacement treaty during the German EU presidency, in the first half of 2007, and agreement on the main points of the new treaty was reached at a summit in June that year.
Negotiations continued behind the scenes over the following months, until a final draft was agreed by the leaders of the 27 member states in October 2007.
Why was the constitution dropped?
France and the Netherlands said they would be unable to adopt the constitutional treaty without significant changes, following the 2005 referendums.
The UK also pressed hard for a modest "amending treaty", which could be ratified by means of a parliamentary vote, like earlier EU treaties.
Does the Charter of Fundamental Rights feature in the new treaty?
No. There is a reference to it, making it legally binding, but the full text does not appear, even in an annex.
The UK has secured a written guarantee that the Charter cannot be used by the European Court to alter British labour law, or other laws that deal with social rights. However, experts are divided on how effective this will be.
Poland won an opt-out from the Charter, because it insisted on retaining national control over family issues and morality, such as abortion.
The Czech Republic also has an opt-out - secured by the Eurosceptic Czech President Vaclav Klaus as a condition for signing the treaty. He wanted a guarantee that his country would not be exposed to property claims by Germans expelled from the then Czechoslovakia after World War II.
Did any countries seek more opt-outs?
The Irish Republic and the UK currently have an opt-out from European policies concerning asylum, visas and immigration. Under the new treaty they have the right to opt in or out of any policies in the entire field of justice and home affairs.
Dublin also won guarantees that the treaty would not infringe on its sovereignty in the areas of taxation, family issues and state neutrality.
Denmark will continue with its existing opt-out from justice and home affairs, but has the right under the new treaty to opt for the pick-and-choose system.
Was the Irish Republic the only country to hold a referendum?
Yes. Most EU leaders argued that Lisbon merely amended earlier treaties and that there was therefore no need for a referendum.
That position was rejected by the Irish No camp and the opposition Conservative Party in Britain, as well as by many Eurosceptics across the EU.
The Irish Republic was obliged to hold a referendum because of an Irish Supreme Court ruling in 1987, saying that any major amendment to an EU treaty entails an amendment to the Irish constitution.
Before the second Irish referendum, Dublin won guarantees that Lisbon would not affect Irish sovereignty in key areas that the No camp had highlighted.
What happens next?
• The High Representative for Foreign Affairs - Baroness Catherine Ashton from the UK - is getting to grips with the EU foreign policy portfolio. Her new EU diplomatic service is taking shape, drawing in specialists from the Commission and national administrations;
• The new President of the European Council - Belgium's Herman Van Rompuy - is chairing EU summits, though many EU ministerial meetings are still being chaired by the country holding the six-month presidency - currently Hungary;
• The new European Parliament was elected in June 2009 under the existing Nice Treaty. So there are 736 MEPs - down from the previous 785. Under the Lisbon plan, the number will be fixed at 751;
• A new 27-member European Commission took office in February 2010, after a three-month delay, having won the backing of MEPs;
• Some extensions of qualified majority voting in the European Council are already in place, but plans to redistribute voting weights have been delayed until after 2014.