The aim of this article is to provide an overview of the role of parliaments in complex unions. For the purpose of analysis and balance four different unions—the USA, the European Union, the Russian Federation and Mercosur—currently at different stages of evolution, have been selected. For each union, the parliamentary dimension is examined against its historical (constitutional) background. Reference is then had to the parliament's composition followed by a study of how it interacts with other institutional players at the federal level, with particular emphasis on the executive. This is achieved by a description of the parliament's participation in its classical legislative and supervisory roles. T h e discussion then examines the relationship of the ‘federal’ centre with the ‘sub-federal’ units of the union, again focusing on the parliamentary dimension of these relations. The short conclusion attempts to compare the roles of the four parliaments in three key areas of competence—government formation, legitimization and scrutiny and influence, focussing in the latter on the use of committees and the role of lobby groups.