The legal framework applying to cooperatives is not harmonised in the European Union. Diverging from other legal systems, the Portuguese legal system made the co-operative principles part of the Portuguese Constitution, giving them a mandatory nature, protecting the co-operative identity.As evidenced in the PECOL project, demutualisation, digital transition, diversity, the professionalisation of management, transparency, prevention of conflicts of interest and accountability, investor members and plural voting are the main challenges addressing the principle of democratic member control in the 21st century. The article concludes that these challenges must be addressed in the framework of the principle of democratic management of members. Plural voting can be a response to the heterogeneity of members and that the admission of investor members can contribute both to the professionalisation of co-operative management and co-operatives' sustainability. Portuguese regulation preserves the essential core of the principle of democratic member control.The article's main contribution is to disseminate internationally a legislative experience of hard law regarding the nature of the co-operative principles. It conveys the message that this political-legislative option has not slowed down the adaptation of co-operatives to new challenges.