Abstract Article 155 of the Spanish Constitution (SC) imports a rule with a long tradition in German federalism: federal execution (Article 37 GG) which has no precedent in Spain. The dreaded Article 155 of the Constitution was applied for the first time in Spain due to the initiation of a process of secession in Catalonia in autumn 2017. This article analyses the constitutional framework of Article 155, its practical dimension in this first case, and the intense controversy caused by its application (particularly some of the more drastic coercive measures) on which the Constitutional Court will have to rule. This reconstruction of the (political and legal) situation seeks to provide tools to understand why, to what end and how Article 155 was applied. It also looks into the questions this application has raised, where difficulties in overcoming the impasse lie and why solving the serious Catalonian crisis requires more than just legal responses.