Comparing the EU law principle of loyalty with international law good faith and the duty of federal good faith in German constitutional law (Bundestreue), this article contributes to the discussion on the nature of the EU legal order and its relationship to international law more generally by finding that EU loyalty is in essence a specific incarnation of the international law principle that treaties are to be interpreted in good faith. At the same time, it challenges the assumption that international law good faith differs fundamentally from federal good faith. To this end, the article points at historical links between both, and posits that good faith is in essence a principle of constructive interpretation, the strictures of which increase with the level of integration of the legal order in which it is applied.
