DOWNLOAD REPORT Nuclear “irreversibility” is not a new challenge; it has been a component of nuclear arms control and disarmament efforts for decades. Irreversibility entered the Nuclear Non-Proliferation Treaty (NPT) lexicon in the 2000 NPT Review Conference Final Document, in which state parties explicitly linked irreversibility to the verification of fissile material removal, as well as broader arms control and disarmament obligations. The concept appeared again in the 2010 NPT Review Conference action plan in tandem with verification and transparency of disarmament measures. In the 2010 action plan, verification and transparency were a means to ensure irreversibility, specifically in the context of fissile material removal. Recently, in the lead-up to the 2022 NPT Review Conference, the United Kingdom and Norway launched an initiative involving government and nongovernmental experts to engage with questions about what nuclear irreversibility might look like and how it might happen. The initiative included a March 2022 Wilton Park conference, which described the initiative’s goal as “to start an international dialogue to better understand and identify requirements” for irreversible nuclear disarmament (IND). Irreversibility in practice, however, remains relatively underexplored, and future multilateral efforts to implement IND will have to address a series of challenges and questions: Do states need a shared vision of an irreversibly disarmed world? How might future verification overcome contemporary and familiar hedging/latency issues? What is the relationship between political, legal, and technical factors in the pursuit of irreversibility? What are alternate frameworks for thinking about IND? Should non-NPT states be involved in IND and at what stage? To help explore these questions, the Project on Nuclear Issues commissioned a series of think pieces from a diverse range of authors and regions to explore potential approaches and challenges to IND. The series interrogates narratives about irreversibility and disarmament and theorizes about paths forward. This publication was made possible by support from the Norwegian Ministry of Foreign Affairs.