A case gets under way today in the European Court of Justice on the extent to which US tech giants based in Ireland, such as Facebook, Google and Microsoft, should be allowed to transfer the personal data of users to the United States.
The case could have major implications for the relationship between the EU and US in the realm of data protection.
It has been referred to Europe's highest court by the High Court following a case brought by Austrian student Max Schrems against Facebook's Irish subsidiary over its transfer of data to the US.
Last year, the High Court referred a complaint by the student to the European Court of Justice.
Mr Schrems had originally taken the complaint against Facebook to Ireland's data protection commissioner.
It came in the wake of allegations by US whistleblower Edward Snowden that US tech multinationals, such as Facebook, had conspired with the US National Security Agency to probe the data of internet users against their knowledge.
At the time, the Data Protection Commissioner said that any data gathered by US tech companies based in Ireland was governed by the so-called Safe Harbour Convention, an agreement designed to ensure that European data was treated in the US according to the appropriate privacy standards.
When the Commissioner dismissed the complaint it was referred to the High Court, which in turn referred it to the European Court of Justice in Luxembourg.
There will be enormous interest in today's opening hearing, with member states, the European Parliament, and lawyers for tech companies expected to make submissions.
The final judgment on the case is not expected until the end of the year.