This chapter considers the sources of the law of weaponry and discusses matters critical to understanding it. Taking the traditional approach, in addition to general principles of law recognized by nations, the sources of the law consist of customary law and of treaty law, the latter referred to as ‘conventional law’. The chapter looks at customary law, which is, according to Article 38(1)(b) of the Statute of the International Court of Justice (ICJ), the law applied by the court as including ‘international custom, as evidence of a general practice accepted as law’. The chapter discusses what does, and respectively does not, comprise state practice and then looks at treaties, how they are made and interpreted, how states explain their understandings of them and related matters. Individual sections then address the status at law of the ICRC Study of Customary International Humanitarian Law and of the UN Secretary General’s Bulletin.