In its capacity as the “Guardian of EU law”, the European Commission has the power to start an infringement procedure if it finds that individual Member States have failed to fulfil their obligation to have properly transposed the provisions of a Directive into national law by the time of expiry of the transposition deadline. In the end, the Commission may bring disputes on alleged infringements before the Court of Justice of the European Union (ECJ). As compared to preliminary rulings, infringement procedures contain two additional elements that may be decisive for the ECJ’s decision: formal procedural aspects and the onus of proof. In this article, the author discusses in particular the onus of proof in infringement procedures.