This chapter addresses the indirect enforcement of European law through the national courts. The core duty governing the decentralized enforcement of European law by national courts is rooted in Article 4(3) TEU: the duty of ‘sincere cooperation’. The chapter explores two specific constitutional principles that the European Court has derived from the general duty of sincere cooperation: the principle of equivalence and the principle of effectiveness. Both principles have led to a significant judicial harmonization of national procedural laws. The chapter then considers the State liability principle, and looks at the procedural bridge that exists between national courts and the European Court of Justice. For from the very beginning, the European Treaties contained a mechanism for the interpretative assistance of national courts: the preliminary reference procedure.