This paper tries to evaluate the contributions of the theoretical researches of the “new political economy” and the “international political economy” in the field of European integration. The former starts from the principle of individual rationality assuming as exogenous the identities and the preferences of agents; the latter analyses the institutional and collective dynamics, in particular the interstate conflicts and compromises or the constructivist power of institutions over the identities and preferences of the agents. The article shows why and how the two political economics must be taken together, in particular to analyze three topics: the institutional products; the organization of powers; the fiscal and social competition.