Approaching the field of personal data protection through a line of inquiry centred on principles, both at a general level and in their specific articulation under the GDPR, opens up research opportunities that have hitherto remained relatively unexplored. It is within this perspective that the inquiry developed in these pages is situated. After outlining the significance and scope of the principles applicable to personal data protection, including their various manifestations in light of both the evolution of the legal order and the relentless advance of technological progress, this study focuses, within that framework, on the principle of solidarity, a notion that is often invoked loosely and excessively. Its aim is to assess the principle’s actual significance and operative reach, thereby pursuing, in this specific field and in intellectual continuity with them, the line of inquiry masterfully developed by Stefano Rodotà and Guido Alpa. Following an examination of the doctrinal debate and of these scholars’ contributions on the principle of solidarity, the research seeks to recast the discussion within the specific framework of personal data protection. It is also directed towards examining the ways in which the relevant authorities – both the independent administrative authority entrusted with supervisory functions in the field of personal data protection and the European and national courts – draw upon the principle of solidarity in legal reasoning and in identifying solutions appropriate to the case at hand. From this perspective, the study seeks to investigate how, in practice, the ‘diritto vivente’ performs, within the specific field of data protection, the ‘ordering’ and ‘adaptive’ functions of the principle of solidarity as a normative principle.