The promise by the European Central Bank to do "whatever it takes" to save the euro is being challenged in Germany's constitutional court.
The ECB's vow to buy up the bonds of eurozone countries if they come under severe pressure has been credited with arresting the eurozone crisis.
There are fears that a court ruling against it could reignite market panic.
The head of Germany's own central bank is expected to tell the court that the policy breaches Germany's constitution.
Bundesbank President Jens Weidmann believes it is illegal because there is no limit to Germany's spending liability under the scheme, and that it is effectively a back-door way of providing loans to other countries' governments.