For a new edition of a work of this kind to be called for only five years after the original was published is an indication of the extent to which its subjectmatter is still in continuous development. This is not to say that new sources are being discovered or devised: one of the contentions advanced in this book is that in this respect international law is fully developed, that what may appear to be a new source of law will turn out, on inspection, to be a variant, or a derivation, of one of those classically recognized. But that does not mean that the law itself deriving from those sources is static; it is continually called upon to apply to new questions, or to mould itself to new requirements, and these may be revelatory of particular aspects of sources- theory. It is one recognized source in particular that goes on requiring or attracting the attention of international scholars and judges: customary international law, to use the form of reference preferred by the International Law Commission; also referred to in the past simply as ‘custom’. Despite a very visible presence in international relations of major multilateral treaties and other documentary material (some in effect codificatory, e.g., ILC reports and conclusions), custom continues to attract the most attention, and this for two reasons. First, being a more flexible concept and process than treaty law, it is continually being revised and re- examined in practice and, significantly, in judicial decisions; and secondly, it is the most fertile field for the enunciation of new theories of how it operates, or how it ought to operate. These, or some of them at least, whether one agrees with them or not, demand to be noticed, if not necessarily discussed, if a study of sources is to make a claim to completeness.