Lower courts may have to refer questions of law to the Federal Constitutional Court (Bundesverfassungsgericht) in Karlsruhe, as a matter of German constitutional law, or to the European Court of Justice in Luxembourg, as a matter of European law, to obtain a binding interpretation of that particular question of law. But nowadays, with national and European law being closely intertwined, it may not be so easy to tell where to send your questions to.
Verfassungsblog has an English language post about the most recent guidance, issued last week, from the Federal Constitutional Court on where to go.
In short: If in doubt, whether German law is in violation of European law, the court must first establish whether the European legislation left some leeway to the national law maker. If it did, the court must first go to Luxembourg, irrespective of whether it is a “court or tribunal of a Member State against whose decisions there is no judicial remedy under national law” or not. Only once Luxembourg has ruled that the implementation of the European guidelines by the German law makers did not step outside the corridor of interpretation permitted under European law, the court may then turn to Karlsruhe with any remaining concerns about the compatibility of such an interpretation with the German constitution.