Insiders pose a significant threat to nuclear/radiological security, as they generally possess access rights which, together with their authority and knowledge, grant them far greater opportunity than outsiders to bypass dedicated nuclear and radiological security elements. In 2016, recognizing the international community’s need to address the insider threat, the United States sponsored IAEA Information Circular 908 (INFCIRC/908) - Joint Statement on Mitigating Insider Threats, which now boasts 31 Member States and INTERPOL as subscribers. INFCIRC/908 seeks to build awareness of and acknowledge that the insider threat is credible and that insiders pose a serious threat to the security of nuclear and other radioactive materials. The United States Department of Energy and the Belgian Federal Agency for Nuclear Control, in cooperation with the International Atomic Energy Agency (IAEA), convened the inaugural International Symposium on Insider Threat Mitigation in Brussels, Belgium during 12-14 March 2019. The purpose of the symposium was to support INFCIRC/908 and develop and foster an integrated insider threat mitigation strategy for the nuclear security domain. One of the outcomes of the symposium was the organisation of the “International Working Group on Insider Threat Mitigation.” The International Working Group, formally announced at the 2020 IAEA International Conference of Nuclear Security, is Co-chaired by the United States and Belgium and supported by a Steering Committee of ten subscribing Member States. While the pandemic restricted travel and in-person meetings for most of 2020, the International Working Group hosted a virtual Steering Committee meeting in October 2020 and a second virtual meeting in January 2021. Five focus groups, will conduct in-depth analysis and develop innovative products to showcase challenges, best practices, and resources. The focus groups include (1) National Policy and Regulatory Frameworks, (2) Security Culture, (3) Cybersecurity, (4) Trustworthiness and Reliability, and (5) Physical Protection and Technical Measures. These focus groups will bring subject-matter experts from different countries together with experts from private and public organisations and academia. Collectively we will establish a global community of practice to advance insider threat mitigation for nuclear and other radioactive materials security, identify concrete needs, and produce practical tools for tangible and lasting results.