[en] The notion of the nuclear threshold first appeared in reference to those States beyond the five Nuclear-Weapon States recognised by the Nuclear Non-Proliferation Treaty (NPT) that had acquired or were in the process of acquiring nuclear weapons. Historically, the first States to be dubbed threshold States were Israel, India, and Pakistan, but the term has since been extended, at least in expert analytical circles and in certain official declarations, to include other countries, both States Parties and non-States Parties to the NPT, such as South Africa, Iraq, North Korea, and, more recently, Iran. Aside from the fact that they constitute or have constituted a scenario of extremely advanced nuclear proliferation, these different countries have very little in common. Situated in singular geopolitical contexts, these countries' specific political/ strategic developments have for the most part provoked nuclear crises, to which the international community has sought to respond via an appropriate diplomatic approach (with the use of force remaining the exception to the rule), with contrasting results. Moreover, with the exception of Iraq and South Africa, the exact extent of the technical and operational development of these States' military nuclear capabilities remains unknown, a point that clearly illustrates the vague nature of the nuclear threshold concept. Indeed, this concept is very much multidimensional, given its simultaneous political, military, diplomatic, strategic, industrial, scientific, and technical characteristics. It also refers to discourses or deterrence postures that vary from one proliferating State to another, which thus require specific interpretation, analysis, and responses to the ensuing crises, which are always likely to weaken the global non-proliferation regime. In this context, an overarching review of the concept and its implications would be extremely useful, all the more so given that no study of this kind has appeared in academic literature to date. Such an approach would consist of going over the different dimensions of the nuclear threshold concept one by one and responding to a certain number of questions raised by an initial review. These questions can be divided into four categories: Those relative to the concept itself: is the threshold notion a useful concept? What does threshold mean exactly? Do several thresholds exist? What does a military nuclear capability entail? Those relative to the concept's implications for international law and arms control: are thresholds to be found in existing international instruments? Is the lack of a definition of what constitutes a nuclear weapon in the NPT problematic? Could new obligations be established that would amount to a de facto threshold (for instance, limiting States' rights under Article IV of the NPT)? Those relative to the concept's political implications: can or should a threshold be associated with a red line? How is the threshold notion perceived by different States? Those relative to the potential regional or global consequences of crossing a threshold: what can or should the international community's reaction be? Does the nuclear non-proliferation regime possess the necessary resilience to react or adapt? In various respects, the last category of questions is the most crucial. As suggested above, the emergence of a new nuclear-weapon-capable country has foiled the international community on several occasions. Is this kind of occurrence inevitable or can other diplomatic strategies, which could be coercive where necessary, be found? (author)