This publication offers practical guidance to Member States on the role of expert support in assessing alarms and alerts for the detection of nuclear and other radioactive material out of regulatory control. It illustrates how experts can help determine hazards, locate and identify materials, and provide advice for decision making when initial assessments are inconclusive. This publication is intended for competent authorities and organizations such as research institutes, regulatory bodies, law enforcement and others involved in nuclear security detection. By applying this guidance, Member States can improve their detection systems and strengthen collaboration to address nuclear security challenges.