Armed insurgents often establish their own courts in territory under their authority. Is it possible to view such rebel courts as embodiments of the rule of law and, in turn, legal under accepted norms of public international law? In the case of fundamentalist Islamic insurgents, a further and significant challenge reflects their rejection of state sovereignty and, as a consequence, the normative legitimacy of public international law. Driven by a desire to eschew ‘modern’ interpretation of the Qur’an and Sunnah, the Islamic State and the Taliban invoke institutions and norms from the early phase of Islam to sanction a limited number of crimes with extreme brutality. Beyond these similarities, however, each group structures its judicial governance in very different ways and deploys it for different purposes. Islamist rebel justice is analysed in light of international law, as part of a book-length study on the administration of justice by non-state armed groups in conflict zones.