These guidelines have been developed to support countries to relieve pain in their paediatric populations. These guidelines address persisting pain in children caused by conditions such as cancer, HIV/AIDS, sickle-cell disease, burns, trauma, and phantom limb pain. Recommendations -- developed following a careful and transparent appraisal of available evidence -- are presented for the pharmacological treatment of mild, moderate and severe pain. The guidelines include chapters on the various systems used to classify pain and on the evaluation of pain, which reviews the available tools for routine pain measurement in children. Provision of sustainable pain relief within health care systems is covered in a separate chapter. Since morphine and other opioid analgesics -- required for the relief of moderate-to-severe pain in children -- are listed under the 1961 UN Single Convention on Narcotic Drugs, the new guidelines include an annex explaining the Convention's requirements for the handling and procurement of opioid analgesics for the relief of pain.