The foundational treaty of the global nuclear order, the Nuclear Non-Proliferation Treaty (NPT), does not define what constitutes a nuclear weapon and therefore what activities, technologies, and materials should be regarded as evidence that a state is seeking to acquire nuclear weapons. This lack of definition exacerbates the nonproliferation challenge of distinguishing between legitimate nuclear activities (be they peaceful or military applications such as naval propulsion) and illegitimate ones (namely, those oriented toward nuclear weapons). This challenge, in turn, exacerbates the difficulty of promoting the peaceful spread of nuclear energy while, at the same time, preventing weapons proliferation. This grant supports an initiative by the Carnegie Endowment for International Peace to build an international consensus around how to distinguish between legitimate and illegitimate nuclear activity. The Carnegie team will convene policymakers, regulators, and technical personnel from the five permanent member countries of the UN Security Council - China, France, Russia, the United Kingdom, and the United States - for a series of non-political meetings to discuss national perspectives on what constitutes illegitimate nuclear activity, weigh the costs and benefits of potential frameworks, and identify areas for further technical analysis.