In the referendum on Sunday, nearly 10 million Greeks will vote 'Yes' or 'No' to more austerity measures in return for bailout funds.
Greek Prime Minister Alexis Tsipras called for a referendum, being organised in just nine days, on a deal with European governments, the European Union (EU) and International Monetary Fund (IMF).
The last referendum held in Greece took place 41 years ago, in 1974, when voters abolished the monarchy.
What is the question?
"Should the deal draft that was put forward by the European Commission, the European Central Bank and the International Monetary Fund in the Eurogroup of June 25, 2015, and consists of two parts, that together form a unified proposal, be accepted? The first document is titled 'Reforms for the Completion of the Current Programme and Beyond' and the second 'Preliminary Debt Sustainability Analysis'."
It's quite a mouthful and has been slammed by critics as confusing and overly technical.
Greece's creditors had asked for reforms in exchange for extending the country's bailout deal until November. But when negotiations between the government and creditors collapsed last week, the extension was refused and the bailout ended as scheduled on June 30.
Greeks are therefore being asked to vote on an offer which is no longer on the table.
Those against the proffered deal will vote 'No', those in favour 'Yes'.
Why a referendum?
After five months of negotiations, Tsipras surprised his European colleagues on the night of June 26-27 by announcing a referendum. The government has said it believes a 'No' result would strengthen Athens's hand at the negotiating table and ensure it lands "a better deal".
Who hopes what?
The coalition government, made up of Tsipras's radical left Syriza party and the Independent Greeks (ANEL), are campaigning for a 'No' vote, backed by the neo-Nazi Golden Dawn party.
Opposition parties New Democracy (centre-right), Pasok (centre-left) and To Potami (centre-left) want a 'Yes' vote to win.
What's at stake?
European leaders have said a 'No' victory would essentially mean Greeks voting to ditch the euro, or at the very least a plunge into the unknown which could damage Greece's relations with the eurozone.
A 'Yes' victory would be a severe blow to the government and would undermine its legitimacy. Finance Minister Yanis Varoufakis has already said he will step down if the 'Yes' camp wins.
There are 9,855,029 people on the electoral register. Voters will be able to cast their ballots on Sunday in 19,159 polling stations across Greece.
To encourage people to vote, there will be discounts on train and bus tickets as well as domestic flights.
As in every election, one million Greeks living abroad will have to return to the country if they want to vote.
How much would it cost?
The interior ministry has said the referendum will come in under 25 million euros ($27 million), less than half the amount spent on the general election in January which brought the Syriza party to power.
What time will the result be known?
Polling stations will be open from 7am local time (0400 GMT) until 7pm (1600 GMT). The first results are expected around 9pm (1800 GMT). It depends how close the gap is between 'No' and 'Yes' as to when Tsipras will cry victory -- or concede defeat.