Whether it is a Yes or No vote, the referendum result will be followed closely around the world, especially regarding the possible "domino effect" in Europe.
Scotland's decision is of particular interest to Flemish nationalists in Belgium and the Catalans in Spain.
ITV News Europe Editor James Mates reports from Brussels:
Former European Commissioners have warned Scotland that its future in the EU is "best secured by staying together in the UK".
People from the President of the European Commission to the greatest living expert on EU law, to the Spanish Europe Minister and the Leader of the Socialists in the European Parliament all agree: if Scotland leaves the UK it also leaves the EU.
There would - and could - be no "seamless transition".
The contrast to those who claim, without any evidence or proof, that somehow it will be "all right on that night", could not be more stark.
The European Union will press ahead with implementing new sanctions on Russia later today despite a shaky ceasefire in Ukraine, the European Commission said.
Details will be published in the EU's Official Journal on Tuesday at the latest, Commission spokeswoman Pia Ahrenkilde-Hansen told reporters.
Incoming European Commission president Jean-Claude Juncker appears to have sidelined the British representative in his new EC college lineup.
Leak of line-up in new European Commission suggest big jobs for E Europe, small roles for UK+France, Germany gets what it wanted.
The website Euractiv said it has seen a draft version of a presentation which suggests that Britain's Jonathan Hill has been assigned to the energy and climate change brief despite David Cameron asking for a super-commisioner.
Mr Hill will report to Latvian Valdis Dombrovkis, who is the vice president for energy.
The leak also suggests that Germany has been assigned the crucial trade role while France, which asked for the economy position, has instead been given the competition title.
The incoming president of the European Council, Donald Tusk, has pledged to address the concerns of Britain in his new role.
He said the EU and he personally would take on the concerns voiced by Britain about the EU. "No reasonable person can imagine the EU without the UK," he said.
Prime Minister David Cameron has promised a referendum on continued EU membership by 2017.
Poland's prime minister Donald Tusk has been chosen by European Union leaders to become the new president of the European Council.
The top official will chair EU summits and mediate between the 28 governments for the next two-and-a-half years.
Federica Mogherini, the Italian foreign minister, has also replaced Catherine Ashton as the EU's foreign policy chief.
Ukraine's President Petro Poroshenko has said that he believes the crisis in the east of the country is very close to the point of "full-scale war".
He said that EU leaders agreed to prepare new sanctions against Russia, which could be implemented depending on the progress of his peace plan.
He added that he hopes a meeting with Russian amd EU representatives on Monday can bring about a ceasefire.
A draft statement from EU leaders meeting in Brussels calls for officials to urgently prepare possible new sanctions against Russia over the crisis in eastern Ukraine.
They will ask the EU's executive Commission and the EU's diplomatic service to work together "to urgently undertake preparatory work" on further sanctions.
Russia is at war with Ukraine and so effectively at war with Europe, Lithuanian President Dalia Grybauskaite has said, calling on Europe to supply Kiev with military equipment.
It is the fact that Russia is in a war state against Ukraine. That means it is in a state of war against a country which would like to be closely integrated with the EU. Practically Russia is in a state of war against Europe. That means we need to help Ukraine to ... defend its territory and its people and to help militarily, especially with the military materials to help Ukraine to defend itself because today Ukraine is fighting a war on behalf of all Europe.