Bare store shelves and empty gas tanks. Hyperinflation. Riots in the streets.
That is the dark view of what could happen if Greece is forced to abandon the euro and adopt its own currency.
Then there is the brighter scenario: A surge in exports. An influx of tourists. An end to five years of economic hardship.
Those stark alternatives — or perhaps more realistically, something in between — are what economists bat back and forth as they consider the chain of events that could begin as soon as Monday, if Greece cannot reach a deal with its creditors and is forced to introduce some new form of money.
To some economists, a new currency would turn Greece into a de facto failed state, creating a humanitarian disaster that would take years to repair.
Others argue that the only way Greece can be competitive in world trade is to use its own currency and free itself from the economic straitjacket imposed by eurozone membership.
Lost in the debate, though, is much sense of just how complicated it is for a country to change from one currency to another, especially considering that there is no indication the Greek government has made any serious preparations for such an event.
Printing new money and making adjustments to A.T.M.s is a huge logistical task that typically takes years of preparation. Even more difficult, experts say, is the task of imbuing the new currency with enough credibility that foreign suppliers and Greeks themselves will accept it as a form of payment.
And ironically, some economists say, the best way to bolster a currency's value is for the government to impose exactly the kind of austerity budgets that Prime Minister Alexis Tsipras of Greece has promised to abolish.
"You need restrictive fiscal and monetary policy, which is something the Greek government does not want to do," said Adalbert Winkler, a professor at the Frankfurt School of Finance and Management.
The pressure on Athens to introduce a substitute for the euro grows with each day that Greek banks remain closed, suffocating the economy.
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That day could some as soon as Monday. If there is no agreement between Greece and its main creditors — the other eurozone governments and the International Monetary Fund — the European Central Bank would then be obliged under its rules to end emergency lending to Greek banks. Those banks would then fail, and the country would have no access to fresh euros and no functioning system for processing transactions.
Currency changeovers are relatively common. In January, for example, Lithuania replaced its own currency, the litas, with the euro. SAP, the German maker of software for businesses, even has a help portal for companies operating in countries that are joining the euro.
But Lithuania's switch came after a decade of preparation, in an atmosphere of relative calm and solid economic growth.
Greece would be going the other direction, leaving a widely circulated, strong currency for an untested new one, during a period of political strife and economic downturn. The assumption is that Greece would use the name of its pre-euro currency, the drachma. But even that detail has yet to be decided.
A likely first step, economists say, would be for the bankrupt government to issue i.o.u.s to civil servants, pensioners and government suppliers because it would be unable to pay them in euros. The government might promise to repay the i.o.u.s in euros at some later time.
There is precedent for i.o.u.s. California issued so-called warrants in 2009 to pay tax refunds and other bills during a budget crisis. But California banks did not close as they have in Greece, and Californians did not need to worry that they might later be repaid in a new currency that did not yet exist.
In Greece, though, desperate pensioners and companies would probably try to use the i.o.u.s to buy other goods and services, turning them into a de facto currency. The value of the i.o.u.s compared with their face value might well plummet because of doubts about the government's ability to repay.
Eventually, the Greek government would have to issue a new drachma or other currency, which it would use to honor the i.o.u.s.
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While electronic transactions could be converted fairly quickly, it takes at least six months to print and distribute a new currency, said Andrea Nitsche, a spokeswoman for Giesecke & Devrient, a company in Munich that prints money for governments and supplies money-printing machinery.
And that is under the best of circumstances. Typically, the process takes 12 to 18 months, Ms. Nitsche said, because of the time needed to design a new currency resistant to counterfeiting and to do other preparatory work.
Vending machines would also need to be converted to the new bank notes and coins. That may seem trivial, but it could have a major impact on daily life for ordinary Greeks who, for example, might not be able to buy subway tickets from vending machines during a transition period.
While the European Central Bank has responsibility for printing euros, Greece still has a mint in the Athens suburb of Halandri that prints 10-euro notes and could theoretically produce drachmas.
As an interim solution, the mint could simply stamp the 10-euro notes to indicate that they are now drachmas issued by the Greek government, said Dirk Meyer, a professor at Helmut Schmidt University in Hamburg who has written extensively about how a changeover might unfold. The Czech Republic used stamps to differentiate its bank notes from those of Slovakia when the two countries formerly known as Czechoslovakia split in 1993.
The far bigger challenge, Professor Meyer said, would be for the Greek government to convince people that the new currency was a reliable store of value.
Typically, when countries face severe devaluation of their currencies, they slash government spending to eliminate fears of runaway inflation. But such tight-fistedness would be incompatible with election promises by Mr. Tsipras and his Syriza party to restore cuts in pensions, rehire civil servants and reverse the austerity policies of recent years.
Another option is to peg a weak currency to a strong one, like the euro. But that would defeat the purpose of having a new currency.
Skeptics say they doubt that the leftist government could resist the temptation to simply print drachmas wholesale, leading to astronomical inflation rates of the kind that afflicted Serbia and Bulgaria in the 1990s. The new currency would become all but useless for buying imported fuel, food and other essentials from foreign suppliers. Severe shortages could lead to civil strife.
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Professor Winkler and others questioned whether the Greek government, many of whose politicians have never before held public office, has enough experienced people to manage a currency swap.
"Introducing a new currency and keeping its value stable requires some technical and conceptual abilities," Professor Winkler said. "You might have doubts whether this will work."
Under any scenario, it is almost inevitable that the new drachma would plunge in value compared with the euro.
But that is not necessarily a bad thing. Some economists argue that, after a difficult period, Greece would be better off with its own, weak currency.
Exports would surge because Greek products like olive oil and feta cheese would become bargains for people with euros, dollars and other stable currencies. Even more visitors than now would flock to Greece's tourism hot spots, taking advantage of rock-bottom hotel prices — or so the argument goes. Greeks who had stashed euros in foreign bank accounts or under their mattresses would suddenly be comparatively rich and might use their hard currency to go on a spending spree.
"A currency reform is a costly end — but a new beginning," Jörg Krämer, the chief economist of Commerzbank, wrote in a note to clients on Thursday.
Others doubt whether things would work out so well.
"If we go to the drachma, it's a completely different political and social framework," said Michalis Massourakis, chief economist at the Hellenic Federation of Enterprises, a Greek business trade group.
In that case, he said, the central bank would start printing money "with no end in sight." Mr. Massourakis predicted: "This is going to end in hyperinflation."