This prologue sets the scene by introducing the book’s main thesis that during the debt crisis, in particular, the years 2010-2012, the European Union has gone through a constitutional transformation. The transformation is characterised by a broadening of the currency union’s conception of stability. Its key manifestations are financial assistance for distressed member states and government bond purchases by the European Central Bank. The transformation can be understood through the lens of solidarity as this makes it possible to conceptualise the unity between the member states and to analyse how political leaders managed to uphold this unity during the crisis. And ultimately, it allows for an understanding of why instead of approving the transformation in Pringle and Gauweiler on the merits, the ECJ should have done so through silence.