This book explores the degree to which international law regulates the procedural aspects of security detention or ‘internment’ in non-international armed conflict. It is concerned with the rules governing the grounds on which individuals may be detained, the review procedures that must be provided to detainees, and when detainees must be released. Chapter one begins with an assessment of the distinction between international and non-international armed conflicts. The second and third chapters then consider the rules applicable to internment under international humanitarian law, looking at international and non-international armed conflicts, respectively. The next part of the book explores the role of international human rights law (IHRL), with chapter four discussing the rules applicable thereunder to detention. Chapter five considers the relationship between humanitarian law and human rights law, showing that, only where the common intentions of the states parties to human rights treaties establish a clear agreement as to their inapplicability or modification in non-international armed conflict, will the presumption of the parallel application of humanitarian law and human rights law be rebutted. Chapter six examines some complex questions regarding the practical application of those rules, including the right of states to derogate therefrom, the degree to which they can be ‘read down’ without derogation, their binding nature for non-state armed groups, and their extraterritorial application. The book concludes with a proposal for developing the law in this area; the proposals made are intended to build upon, rather than displace, existing obligations of states and non-state armed groups.