The Cyber Defence Unit of the Estonian Defence League, or the ‘Estonian Cyber Defence League’ as it is widely referred to, is an innovative model for the involvement of volunteers in national cyber defence. It focuses on strengthening the professional cyber defence skills of its volunteer members in order to prepare and enhance support capabilities in a crisis. The Estonian Ministry of Defence has requested the NATO CCD COE to conduct a study of the Cyber Defence Unit model with an overall aim to promote and support similar initiatives in other countries. Specifically, the study outlines the legal context of using volunteers in national cyber defence, identifies the main issues and concerns about the Cyber Defence Unit, and addresses them in a legal context. Given the close connection of the legal issues to policy and organisational matters, and especially the fact that the concept and organisation of the Cyber Defence Unit is in a continuous process of development, it became necessary to extend the scope of the study somewhat and also address policy and organisational issues closely related to the legal aspects.