This publication provides guidance to States for developing a risk informed approach and for conducting threat and risk assessments as the basis for the design and implementation of sustainable nuclear security systems and measures for prevention of, detection of, and response to criminal and intentional unauthorised acts involving nuclear and other radioactive material out of regulatory control. It describes concepts and methodologies for a risk informed approach, including identification and assessment of threats, targets, and potential consequences; threat and risk assessment methodologies; and the use of risk informed approaches as the basis for informing the development and implementation of nuclear security systems and measures. The publication is an Implementing Guide within the IAEA Nuclear Security Series and is intended for use by national policy makers, law enforcement agencies and experts from competent authorities and other relevant organizations involved in the establishment, implementation, maintenance or sustainability of nuclear security systems and measures related to nuclear and other radioactive material out of regulatory control.