Abstract Current understandings of the constitutional effectiveness of EC law emphasise the European Court of Justice's (ECJ's) claims of supremacy and direct effect, and the acceptance of those claims by the national courts. However, the lex posterior problem of EC law in the national legal order—the problem whereby the application of European obligations in the national legal order could be legislated away by subsequent contrary national legislation—has been addressed not by national courts' acceptance of Costa but by national courts' assumption that national legislatures do not intend to legislate contrary to prior European obligations, often developed from separately established national doctrines which assume legislative fidelity to treaty obligations. As such, the solution to the lex posterior problem of EC law in the national legal orders rests on these national legal doctrines combined with pervasive national legislative self-restraint. Political self-control in the Member States supports the European legal order.