This paper deals with analytical and methodological problems which are currently at the core of historical studies about European integration. It also reminds that history of European integration went through different periods which fostered various thematic conceptualizations. Indeed, the “essence” of the European construction, as a diachronic and in fieri process, furthered a renew of the object of study, gradually widening its importance, enriching and renewing historiographical interpretations. The definition of research lines about origins and development of EEC/EU policies, in particular those “second generation” policies promoted since the beginning of the Seventies, is placed in a complex historiographical background. The author shows the reasons to study this topic through a diachronic approach, highlighting that policies are fundamental to understand properly many relevant political and social dynamics at national, infranational, European and also international level.