In the Spring of 2005 the ICRC presented the result of a collective endeavour lasting a number of years. The purpose of its study of Customary International Humanitarian Law (hereinafter, the Study) was to ascertain and state which rules of international humanitarian law (hereinafter, IHL) are part of customary international law. The study is, of course, based on the assumption that in the field of IHL customary law indeed matters. If it did not, the voluminous study would constitute a futile work. But this unattractive perspective does not deter the question: does IHL matter in the field? If so, how and why? And, in addition: what does the Study show as to how customary law is made?

The Study might be meaningful well beyond the scope of IHL. It may convey some lessons on the role and development of customary law in current international relations.