This paper examines from a legal perspective the modes in which EU agencies partake in the enlargement process. In so doing, first the main forms in which agencies become involved in EU enlargement are explored. Next, EU agencies’ forms of participation in the enlargement process are subjected to a legal assessment taking the principle of institutional balance and the delegation of powers within the EU as main yardsticks. In this context, the legal nature of such forms of participation and related instruments, as well as some of the relevant legal problems they might raise are discussed. The paper examines these issues both with regard to the wider category of EU agencies and by focusing on the European Aviation Safety Agency (EASA), an agency actively involved in the enlargement process, in order to provide a concrete illustration of how agencies’ involvement works in practice. It is argued that, apart from the formalized cooperation instruments that could result in legally binding agreements under international law, the forms in which EU agencies partake in the enlargements process do not create insurmountable legal and practical problems. As for the more controversial category of ‘binding agreements’ concluded by EU agencies, objections can be raised in view of the rather conventional reading by the CJEU of the balance of powers between the EU institutions, in particular with regard to Union’s treaty-making. Arguments are advanced for an alternative route based on a dynamic interpretation of the institutional balance and of the delegation of powers within the EU.