Italians have gone to the polls in a referendum which could spell the end of Matteo Renzi's reign as prime minister - and spark political turmoil.
Roughly 51 million Italians are eligible to vote "yes" or "no" to plans to drastically reduce the role of the upper house Senate, and with it streamline a complicated legislative system.
Renzi, Italy's youngest prime minister and leader of the centre-left Democratic Party, has staked his political career on the result by pledging he will resign if the country votes "no".
In such a scenario, Italy could be plunged into political turmoil, which could well have ramifications for the rest of Europe.
Italian populist parties - including the anti-immigrant Northern League - are also waiting in the wings and banking on Renzi losing the referendum.
Financial markets and Europe's politicians fear victory for the opposition "no" camp could trigger political instability, which might cause renewed turmoil for Italy's battered banks.
Such a situation could push the euro zone towards a fresh crisis.
With all the opposition parties lined up against the reform, a victory for Renzi would be a surprise.
Opinion polls suggest he faces defeat.
The "no" campaign has been led by the the anti-establishment Five Star Movement, hopeful of a similar outcome to Britain's referendum on EU membership.
Polls will close at 11pm local time (10pm GMT), with the results due shortly after midnight.