The problem: Greece's huge debts, about 340bn euros (£297bn; $478bn).
In late 2009, after months of speculation and sovereign debt crises in Iceland and the Middle East, Greece finally admitted its debts were the highest in the country's modern history.
Since then, a 110bn-euro bailout was passed by the eurozone last year and a second bailout of roughly the same size was agreed earlier this year - but not yet passed.
Most observers remain highly sceptical of Greece's ability to ever repay its huge mountain of debt. Talk persists of an unprecedented default or of Greece leaving the eurozone.
Because of the interconnectedness of the European economy, this would cause huge losses for French and German banks.
Thus, though Greece has been bailed out, fears of it running out of money continue to plague investors.
International credit markets remain wary of Greece because of its sovereign debt rating.Ratings: Greece is now considered to be "junk" by the ratings agencies, meaning it has a very high chance of defaulting. S&P has cut its debt seven times since 2009, from A to CC, the third-lowest rung on its rating scale.
ITALYRatings: Italy was last triple-A in 1995. Since then, its rating has been fairly stable near the top of the investment grade rankings.
Ratings: Last at the highest rating in 1992, the Iberian nation has been cut twice since 2009.
Ratings: France was given the top rating by Moody's in 1988, and kept it ever since, despite anaemic growth.
Ratings: Following reunification, the country was given the highest possible creditworthiness by S&P in 1992 and Moody's in 1993.
Ratings: In 2009, S&P lowered its outlook on British debt to "negative" from "stable" for the first time since the agency started rating its public finances in 1978. But the triple-A rating has been affirmed since 1993.
Ratings: The Irish Republic held the highest triple-A rating as recently as 2001. S&P has cut it five times since 2009.
Ratings: Portugal has been cut four times since 2009. It was once triple-A, way back in 1993.