Nuclear or radiological attacks are among the most serious CBRNE (Chemical, Biological, Radiological, Nuclear and Explosive) threats because they can cause mass casualties, widespread contamination, disruption of critical infrastructure, long-term health effects and major psychological and economic consequences. A nuclear attack may involve a nuclear weapon or improvised nuclear device, while a radiological attack may involve the dispersal of radioactive material, such as in a radiological dispersal device (RDD), commonly called a “dirty bomb.” Because such incidents can cross national borders and require highly specialized capabilities, several international agencies and organizations are involved in prevention, preparedness, detection, response, recovery and coordination. The International Atomic Energy Agency (IAEA) is the principal international organization dealing with peaceful uses of nuclear technology, nuclear safety, nuclear security and safeguards. It plays an important role in preventing nuclear and radiological materials from being misused. The IAEA develops international safety and security standards, supports member states in strengthening nuclear security, assists in the detection and control of illicit trafficking of radioactive and nuclear materials, and provides assistance following a nuclear or radiological emergency. Its Incident and Emergency Centre coordinates international assistance and information exchange during nuclear or radiological emergencies. The IAEA also conducts training and capacity-building programmes for countries to improve preparedness for such incidents. The United Nations (UN) has an important coordinating role in international