**The EU’s Lisbon Treaty comes into force on Tuesday following its ratification by all the bloc’s 27 member states.**The treaty is designed to streamline decision-making and give the EU greater influence in world affairs.
It creates two new posts - president of the European Council and a high representative for foreign affairs.
Celebrations are planned in the Portuguese capital later on Tuesday with fireworks, music and speeches from EU leaders.
The BBC’s Jonny Dymond, in Brussels, says the introduction of the treaty may appear low-key but the way decisions are made in the EU will change.
Its supporters say it will make the EU more democratic and more efficient, but critics say it will cede too many national powers to Brussels.
The treaty comes into force almost exactly eight years since it was first conceived.
Foreign affairs supremo
It overcame its final hurdle when the Czech Republic became the last of the member states to ratify it earlier this month.
EU leaders later chose Belgian Prime Minister Herman van Rompuy to be the first permanent European Council President.
The job of foreign affairs supremo went to EU Trade Commissioner Baroness Catherine Ashton from the UK.
She takes over from EU foreign policy chief Javier Solana, who leaves his post on Tuesday after 10 years in the role.
As high representative for foreign affairs, Mrs Ashton will command a more powerful position with a new, and eventually vast, European diplomatic service.
One of the treaty’s key areas is the removal of national vetoes in a number of areas.
These include the fight against climate change, energy security and emergency aid.
Unanimity will still be needed in tax, foreign policy, defence and social security.