This Study is an original empirical account of internment practices by States and non-State armed groups in non-international armed conflicts to find evidence of common behavioural patterns by warring parties, irrespective of status. It focuses on particularly complex aspects of conflict-related security detentions – permissible grounds, procedural guarantees and transfer standards, that remain unregulated under international law. Unlike previous research, this Study is based on a large and robust collection of primary and secondary data. It explores normative frameworks that belligerent parties applied to their internment; examines operational practices to find compliance with as well as discrepancies in implementing relevant internal regulations; and intersects these findings to identify legislative commonalities and proposes a normative system that is realistic to impose on warring parties and feasible to implement. This methodological approach is critical to diagnosing the status of lex lata in practice and is central to developing credible projects on lex ferenda.