1. What is being proposed?
- To strip the second chamber of parliament, the Senate, of most of its powers to block and amend legislation.
- To replace 315 elected Senators with 100 appointees from the regions.
- To transfer some powers currently held by local and regional authorities to central government
The government says the reform will end gridlock in parliament and make it easier to pass difficult legislation. The changes have been designed to complement reform of the electoral system ensuring biggest vote-winner in elections of a parliamentary majority.
Prime Minister Matteo Renzi argues that the reforms will reduce bureaucracy and lead to savings of up to 490 million euros a year in operating costs (including the salaries of senators), according to disputed government figures.
3. Who's on which side?
FOR: Renzi, most of his centre-left Democratic Party and his junior coalition partners. Most business leaders, including Banca D'Italia and Goldman Sachs (not that this will help Renzi in his battle to convince voters he's not part of Italy's 'establishment').
AGAINST: All the main opposition parties - populist Five Star Movement, Silvio Berlusconi's Forza Italia and the far-right Northern League. Some prominent figures in Renzi's own camp including former party secretary Pier Luigi Bersani and former prime minister Massimo D'Alema, many constitutional experts.
4. Will it carry?
It's very uncertain. Last polls before a two-week pre-vote ban gave the No camp a lead of five to eight percentage points, but with many voters - around a third - undecided.
However, the rule banning publication of polls in the finals two weeks means we really don't know how people are feeling right now. Renzi is hoping that a boost from expat voters (who weren't included in most opinion polls, and have already cast their votes) will swing it his way.
5. What happens if Yes wins?
Renzi stays in power, his authority will be enhanced and he can pursue plans for reforms of the education, legal and administrative systems.
He has made some big promises to voters in the run-up to the vote, even reviving the idea of a bridge to Sicily, and in the event of a close result he'd be under a lot of pressure to prove the capability of his government.
6. And if No wins?
Renzi has said he will step aside - however, it's possible that pressure from other ministers will persuade him to stay, due to fears that his resignation could lead to political instability for Italy.
Many observers expect his party to form a new government without him, possibly with Finance Minister Pier Carlo Padoan as prime minister.
Another option is a 'technocratic' government, something Renzi has described as a 'risk' but which other observers, including Britain's The Economist newspaper, have suggested would be the best option for Italy.
While some global media has painted the possibility of a No win as the 'third anti-establishment revolt' of 2016 - following Brexit and Trump's win in the US - this isn't exactly the case. If the No camp wins, things stay as they are in terms of government. Even Renzi has said it wouldn't lead to "any major disasters".
7. Will there be early elections?
This is possible, but seen as unlikely before late 2017 at the earliest. The main reason for that is that Italy will have to sort out its electoral law before having a general election.
Want more details? Here's everything you need to know about Italy's referendum.