This article deals with Constitutional comparativism in action. In contrast to a view in the US-debate that considers Constitutional comparativism to be futile, the Law of the European Union explicitly relies on comparison in the core issue of general principles of EU (constitutional) law. The case law of the ECJ includes constitutional law comparisons that matter. The article deals with questions such as the question whether the comparative efforts lead to better law, whether it is only about subjective views behind a smoke-screen and how the ECJ model of comparative constitutionalism in action affects players and their perspectives in the Member States. The answers help to better understand potentials and limits of comparative constitutionalism.