Chapter 4 discusses the application of Art. 34 TFEU to private rules. Art. 34 TFEU prohibits quantitative restrictions to trade between Member States and measures having equivalent effects. The chapter attempts to understand if, and if so, under which circumstances, Art. 34 TFEU is horizontally applicable also to private measures restricting access to the market. It subsequently discusses the rather limited possibilities and conditions under which a VSS could fall by Art. 34 TFEU, the possible outcome of an Art. 34 TFEU assessment, and possible justification for a breach that can be invoked by private parties. Starting from the EU treatment of private regulation and technical standards, the chapter then studies the interactions between EU authorities and various types of standards in the internal market. It analyses the forms of direct and indirect employment of VSS in EU measures disciplining sustainability in global value chains. The chapter analyses their impact on the allocation of regulatory competences between public and private authority, the implications for substance and procedures of schemes and the trade-barrier effects and schemes’ stringency.