The concept of ‘membership’ plays a key role in the contemporary practice of international humanitarian law (IHL) and counterterrorism law. Membership in a non-State armed group (NSAG), reflected in the function assumed by an individual within that entity, is considered to be a decisive criterion for the purpose of targeting under IHL. Membership in a proscribed/terrorist organisation is widely criminalised at the national level. In situations of armed conflict, both notions often apply to the same conduct, one that criminal law seeks to prevent and punish, and that IHL of non-international armed conflicts (NIACs) neither permits nor prohibits. This chapter discusses the notions of ‘membership’ in national proscription regimes and under IHL of NIACs, and addresses the possible and real areas of tensions that such regulatory overlap creates. It finds that many such tensions do not result from proper conflicts between norms but rather from diverging interests that are at times difficult – but not necessarily impossible – to reconcile.