This report is based on a survey of prison policies in 15 countries and offers a study of the role prisons can play in radicalising people – and in reforming them. Sixteen of the world’s leading experts contributed to the report, which was funded by the governments of Australia, the Netherlands, and the United Kingdom, and carried out in partnership with the National Consortium for the Study of Terrorism and Responses to Terrorism (START), based at the University of Maryland. The report identifies trade-offs and dilemmas but also principles and best practices that will help governments and policymakers spot new ideas and avoid costly and counterproductive mistakes. Among the key findings and recommendations are: 1) the current emphasis on security and containment leads to missed opportunities to promote reform. Prison services should be more ambitious in promoting positive influences inside prison, and develop more innovative approaches to facilitate extremists’ transition back into mainstream society. 2) Over-crowding and under-staffing amplify the conditions that lend themselves to radicalization. Badly run prisons make the detection of radicalization difficult, and they also create the physical and ideological space in which extremist recruiters can operate at free will. 3) Religious conversion is not the same as radicalization. Good counter-radicalization policies – whether in or outside prison – never fail to distinguish between legitimate expression of faith and extremist ideologies. Prison services should invest more in staff training, and consider sharing specialised resources. 4) Individual de-radicalization and disengagement programmes – such as the ones in Saudi-Arabia, Singapore, Indonesia, and other countries – can make a difference. Their positive and outward-looking approach should serve as an inspiration for governments and policymakers everywhere. 5) Even in the best circumstances, however, such programmes complement rather than replace other instruments in the fight against terrorism. They work best when the political momentum is no longer with the terrorists or insurgents.