Some processes of integration include a parliamentary body in their institutional structures. This paper combines basic premises of neofunctionalism and intergovernmentalism to address the origin and further evolution of regional parliaments, including the necessary precautions when applying European approaches of integration to other regions. With these postulates in mind, we outline an explanation of the way in which two regional parliaments, the European Parliament and the Mercosur Parliament, emerged and developed. The combination of approaches allows us to affirm that States are important and powerful actors, but the role of regional institutions and transnational actors cannot be ignored. However, comparative analysis shows that while in Europe the dynamics of the process of integration has feedback mechanisms beyond the control of States, in the Mercosur the influence of national Governments is overwhelming and prevents any “self-reinforcing” dynamics.