Three cases from the common law experience (US and Canada) on the enforcement of prisoners rights through class actions (injunctive and compensatory ones) are herein presented. They are the Plata, Ashker and Brazeau cases, and are analysed in order to highlight possible future uses of the same device in Europe, and specifically Italy. The different regulatory, institutional and interpretative approaches—which have allowed such a type of enforcement, until now unimagined in the latter jurisdiction—are also presented in a critical fashion, with a constant constitutional lens. Furthermore, as stressed in a comparative perspective, numerous blatant and structural violations on the same matters, first and foremost concerning overcrowded prisons, have been found by different apex courts in Italy and against Italy, i.e. by the Italian Constitutional Court and the ECtHR (with the Torreggiani case). Against this backdrop, the possible use of the same device for collectively enforcing the rights of migrants is also discussed.