Résumé : This Master’s Thesis explores the issue of the 2015 refugee crisis, and aims to explain the way in which power relations between the European Council and the other EU institutions influenced EU’s response in terms of policy outcomes. It takes as a case study the temporary relocation Decisions adopted by the Council on 22 September 2015: this package of measures sought to urgently relocate a total of 160.000 individuals from Greece and Italy and redistribute them among EU Member States, in order to avoid a total collapse of national asylum reception systems, of the Dublin system and the Common European Asylum System as a whole. However, the final outcome did not meet the expectations. Extreme polarization due to the sensitivity of the issue, an increased politicization caused by the emergence of populist and nationalistic movements all over Europe, the complex balances of powers existing in a traditionally intergovernmentally-led policy area: all these factors contributed to an escalation of tensions that led to a real solidarity crisis. We will try to understand how we arrived at this point, by analysing the role of the European Council, which is more and more becoming a day-to-day actor in asylum and migration; the development of European Commission and Parliament’s positions and, finally, the rationale put forward by some Member States when justifying their refusal to comply with their legal obligations. The result will be, therefore, an assessment of the European Council’s ‘shadow of hierarchy’ effect, and its impact on the temporary relocation Decisions.