This chapter broadens the analytical scope to the national parliaments of EU member states, the main guardians of the subsidiarity principle in the EU. It reveals an additional dimension of sovereignty conflicts in the context of the EPPO by examining the “yellow card” triggered against the EPPO Proposal by national parliaments. The chapter goes beyond the lack of a direct impact of the yellow card (the Proposal was maintained unchanged) by investigating how it contributed to the increased visibility of particular ideas, but also how individual parliaments impacted the process through other means. The chapter concludes that national parliamentary influence remains strongest when exercised through traditional domestic channels, as EU mechanisms like the yellow card remain limited in terms of feasibility and formal but also informal repercussions.