Cohesion policy (CP) is arguably one of the most important policies of the European Union (EU) and historically one of the most financially significant, today still representing roughly one-third of the EU budget. Beyond its financial dimension, CP is important also because through it the Community/Union has addressed some of its most vital challenges, from economic development to social and territorial cohesion; and has facilitated the attainment of some of its most defining goals, from the completion of the single market to eastern enlargement. It could be argued that all these goals were implicitly and synthetically captured in the original formulation of the Preamble of the Treaty of Rome, whose felicitous formulation of ‘harmonious development’ is contained in all subsequent treaties, albeit with varied emphasis on the expected drivers and the complementary aims of this development. The nature of these drivers and the compatibility between the several objectives simultaneously pursued through CP, hence the many interpretations that can be given of this policy, are the objects of our analysis. This Handbook, therefore, will not only chart what the EU is and does today, but also give a sense of the many aspects of EU integration that have been addressed through CP during its 40 years of existence. In particular, we will seek to highlight how CP has been used to elicit a sense of joint purpose and common direction among the territories of Europe, in an effort to draw them towards increasingly converging economic, social and administrative standards, thus favouring both a sense of community and a spirit of solidarity. Whether or not CP has achieved these ambitious objectives is obviously questionable, as it is questionable whether these were the ‘real’ objectives of this policy. What, according to us, is unquestionable is that CP is a highly political policy that tends to mobilise a large number of institutional and non-institutional actors, at supranational, national and subnational levels.