What is the reform about?
The reform proposed by Italian Prime Minister Matteo Renzi diminishes the role of the second chamber of parliament, the Senate. It revokes the Senate’s right to hold votes of no confidence in the government and cuts the number of Senators from 315 to 100.
If the amendment passes, these 100 senators would consist of 21 regional mayors and 74 regional council heads who will be appointed by regional bodies. The remaining five seats will be selected by the president.
What does the referendum promise?
Renzi’s government says the reform will bring political stability to Italy by streamlining the parliamentary system and making it easier for stable governments to be formed.
However, opposition parties disagree. They suggest the measures would concentrate too much power in the hands of the prime minister.
At the same time, they see the referendum as a chance to defeat Renzi who has vowed to resign if he loses.
When are Italians voting?
The polls opened at 7 a.m. (0600 GMT) and are set to close at 11 p.m. (2200 GMT) on Sunday.
Some 51 million Italians are eligible to vote and full results are expected in the early hours of Monday.
What might be the consequences?
Renzi’s victory might generate a new mandate to pursue reforms he sees as key to unshackling Italy’s creativity from the influence of a self-serving political caste that has exploited institutional weakness to stymie change.
"If we miss this chance it won't come back for 20 years," he warned voters before campaigning was suspended at midnight on Friday.
His defeat might lead the country to a crisis of investor confidence causing the failure of a rescue scheme for Italy’s most indebted banks. This might trigger a broader crisis across the European Union.
The uncertainty over what may happen following the referendum has disturbed European markets, leading Economy Minister Pier Carlo Padoan to attempt to calm fears by saying there was “no risk of a financial earthquake" if people vote against the changes.
Another concern among supporters of the EU that anti-EU politicians such as Beppe Grillo, founder of the populist Five Star Movement, could fill the void left by Renzi if he resigns.