Taking into consideration the entering into force of the Lisbon Treaty by 1 December 2009, the book offers a major new comparison of the European Union (EU) political system with the American one. After a long period of institutional divergence, Professor Fabbrini argues that the process of regional integration in Europe has significantly reduced the historical differences between the democracies on either side of the Atlantic. The EU and the USA are different unions of states and their citizens, but they function according to a similar logic, that of the compound democracy. Compound democracies are based on a multiple separation of powers. This guarantees that any decision is taken through a complex system of checks and balances. As the failure of the George W. Bush’s attempt of imposing the predominance of the President on the other separated institutions shows, also in the USA, as in the EU, the centralization of decision-making power is irreconcilable with the logic of a compound polity. Here the puzzle: how to guarantee the compound nature of the polity without detracting its decision-making capacity and democratic accountability?