This guide was developed in the context of the OHCHR Treaty Body Capacity Building Programme, established by General Assembly Resolution 68/268 to support States parties in building their capacity to implement their treaty obligations, in this case, their obligations under the Optional Protocol. It seeks to respond to key questions frequently asked about National Preventive Mechanisms (NPMs), and to explain the four core functions of the mechanisms - visiting, providing advice, enhancing cooperation and educating - which are key to their effective functioning. It includes checklists and other guidance offering practical tools to aid their performance. It aims to assist both States planning to establish or seeking to strengthen their NPMs, as well as the staff of the NPMs themselves.