This Implementing Guide presents a comprehensive methodology for the development of preventive and protective measures against insider threats to nuclear facilities and nuclear material transport operations of all types. Institutional insiders who are privy to the inner workings of security systems present a unique challenge to the establishment of effective control systems for nuclear material. They generally possess access rights which, together with their authority and knowledge of facilities, grant them far greater opportunity than any outsider to bypass dedicated physical protection elements or other provisions such as safety systems and operating procedures. Furthermore, insiders, as trusted persons, are capable of methods of defeat that are not available to outsiders. This publication provides guidance and measures for reducing these and other risks posed by insiders.