The migration of constitutional ideas is characteristic for the legal integration within the European Union. EU law and domestic law are closely intertwined, enabling permeability between the participating legal orders. This allows for constitutional ideas to migrate between these orders. A prominent examples is the jurisprudence of the domestic constitutional courts establishing limits to the integration process in the form of “constitutional identity”. The identity language has been taken up by an increasing number of member states and transplanted into their respective constitutional and political setting. Whereas this process reinforces legal integration by “harmonizing” constitutional approaches to the interplay between EU law and domestic law, the “transplantational” dimension of this process also comes with challenges: e.g. the variety of understandings of what constitutional identity means in a specific legal order and the purposes this language is used (or misused) for.