These guidelines provide a recommendation on iodine thyroid blocking (ITB), via oral administration of stable iodine, as an urgent protective action in responding to a nuclear accident. This recommendation aims to support emergency planners, policy makers, public health specialists, clinicians and other relevant stakeholders, in order to strengthen public health preparedness for radiation emergencies in WHO Member States as required by the International Health Regulations (IHR) and in line with the international safety standards (GSR Part 7). The scope of the guidelines is confined to public health aspects of planning and implementation of ITB before and during a radiation emergency, such as dosage and timing of ITB administration, adverse effects of stable iodine, its packaging, storage, and distribution. These guidelines supersede the 1999 WHO Guidelines for Iodine Prophylaxis following Nuclear Accidents.