The Lisbon Treaty is the latest and most crucial reform to the European Union.As the successor to the much-debated and innovative Constitution, it represents the culmination of the unification process, for it aims to simultaneously enhance efficiency and cover the democratic deficit inherent in the outdated decisionmaking mechanisms of the past, as a consequence of excessive enlargement and deepened integration. Thus, the first question is the extent to which the Lisbon Treaty renders the Union more democratic and according to what criteria that may be evaluated. Another question is related to the fact that the Treaty is the result of a fundamental debate on the very nature of the European Union: Is the latter to become an authentic political union or a mere free-trade area with secondary political attributions? And will its internal operating rules correspond more to a parliamentary or a presidential regime?The present work proposes to focus on the claim that the European Union as a polity shall become more democratic with the Lisbon Treaty, and seeks to provide answers using representative democracy and parliamentarianism as indicators. To achieve such an ambitious aim, special attention is given to parliamentary bodies, first at the supranational level with the European Parliament as part of the institutional triangle (Commission-Council-Parliament), then at the internal level with the Member States' national Parliaments as part of the framework of existing and new competences (oversight, vertical separation of powers and subsidiarity).The changes brought about by the Lisbon Treaty towards more democracy are not without consequence to its second proclaimed goal, efficiency. It is thus interesting to examine how both are combined and if there is a positive or a negative interaction between the two. Moreover, the very articulation of the sensitive balance between measures for more efficiency and measures for more democracy has a noticeable impact on the European Union as a political system. Will it function like a federal or an intergovernmental system?Although much of the above will undeniably be determined through the practice of the institutions and the overall political equilibrium that will be achieved, the present work offers useful methods and elements of analysis that are an indispensable guide to anyone with an interest in European Union law and politics.