Chapter 5 looks closely into the notion of law of the Union established by the EAEU Treaty. It analyses the sources of law of the Union and proceeds with clarifying their hierarchy and legal force. It also analyses the place of direct applicability, direct effect and primacy in the EAEU pertaining to legal order autonomy. Eventually it turns to the provisions of Member States’ national legislation vis-à-vis EAEU law. The three founding Member States have been deeply involved in Eurasian integration. The process of drafting the EAEU Treaty required alignment with the generally recognised principles of international law, national legislation of Member States, taking into account international experience, but not the least with national constitutions. Therefore, in principle, tensions between the legal orders of the EAEU and its Member States should have been minimized from the outset. This chapter shows that it is not necessarily so in two case studies: Russia and Belarus. The former has had the most controversial encounters with external legal orders, including that of the EAEU, which was eventually ossified in the ensuing constitutional amendments in this regard. The latter has the strictest constitutional limitations, which have direct bearing on the EAEU legal order.