The obligation to establish National Preventive Mechanisms (NPMs) set out in the Optional Protocol to the Convention against Torture (the OPCAT) fundamentally changes how torture and ill-treatment is to be challenged and addressed. It had long been recognized that impartial, independent scrutiny of the treatment of those in detention plays a vital role in achieving this end, and mechanisms for doing so have long existed in various parts of the world and in a number of States. However, there was no comprehensive system and no recognized approach to how such mechanisms might best be established and operate. Moreover the mandates of many of the mechanisms which did exist was often limited and vulnerable to change. Seen as a global system of prevention, it was partial, fragmented and weak. The OPCAT has transformed this. For States parties the system is now comprehensive, cohesive and strong. Clear guidance exists on the establishment and operation of NPMs, backed by legal obligations and linked to the international system of NPMs and the UN Subcommittee on the Prevention of Torture (the SPT), which is the international preventive mechanism guiding and guarding the system as a whole. This Guide sets out and explains the essential elements of this system and the work of NPMs, and is itself an important contribution to the fight against torture. Sir Malcolm Evans Chair of the Subcommittee on Prevention of Torture iv I PREVENTING TORTURE: THE ROLE OF NATIONAL PREVENTIVE MECHANISMS – A PRACTICAL GUIDE