Chapter 6 examines whether the weapons review obligation as formulated under Article 36 could have now attained customary international law status with the consequence that even States not party to Additional Protocol I are equally required to review weapons for their compliance with applicable international law. This issue remains contentious in the legal literature, with some commentators supporting the customary nature of Article 36 and others being less certain of this obligation. The chapter argues that, in the absence of ‘extensive and virtually uniform’ State practice showing that new weapons, means and methods of warfare are reviewed at the earliest stage in the acquisition process as a matter of law, no customary rule mirroring Article 36 exists. The chapter also demonstrates that, contrary to the argument advanced in the expert literature, an obligation narrower in scope – requiring States to review weapons ‘before fielding’ – also cannot be ascertained. Such an argument is simply not persuasive given the lack of required State practice and opinio juris. [164 words]