Germany's Constitutional Court is holding a two-day hearing to assess whether a policy employed by the European Central Bank to calm the bond markets is contrary to German law.
The controversial policy, known as Outright Monetary Transactions, ultimately amounts to an ECB declaration that it will buy the bonds of euro zone countries which come under market pressure.
It was first signalled in London last July, when the ECB President Mario Draghi promised to do “whatever it takes” to save the single currency.
Even though OMT has yet to be used, it reassured investors they would not lose their money and reduced the cost of borrowing for countries like Spain and Portugal.
However 35,000 German citizens have signed a complaint which argues OMT amounts to the ECB funding governments - something which violates German law.
The Constitutional Court's President, Andreas Vosskuhle, said that the recent success of the ECB's bond-buying programme would have no impact on its deliberations.
The court will hear from the head of the German Bundesbank, Jens Weidmann, who opposes OMT and ECB Board member and fellow German Joerg Asmussen, who supports it.
A ruling is some months away.
While the Court cannot simple abolish OMT it could raise significant questions about the policy's unlimited approach to buying bonds.
If that were to happen, it could once again deter investors from buying bonds considered to be risky - including bonds from states like Ireland.
