International humanitarian law derives from two normative sources – treaties and custom. Rules regarding the protection of persons during armed conflict have been developed in a series of treaties and conventions, dating as far back as 1868. However, these rules have two major limitations. First, treaties apply only to states that have ratified them - thus, different states are governed by different rules in different situations. And second, most of these rules were developed with classic inter-state international armed conflicts in mind, and do not apply to non-international armed conflicts which are the vastly more prevalent type in contemporary times. Customary IHL, on the other hand, comprises general state practice accepted as law.

On June 30, 2008, the Series hosted Mr. Jean-Marie Henckaerts, legal adviser in the ICRC's legal division in Geneva. Mr. Henckaerts led the ICRC study on customary international humanitarian law, and together with Prof. Louise Doswald-Beck co-authored the 3-volume report issued in 2006.

In his lecture, Mr. Henckaerts addressed at length the purpose and importance of this immense project, consolidating rules that applied to all parties to armed conflict, and to all types of conflict; the methodology used in determining 161 rules that the study found to have attained customary-law status; some of the dilemmas with which the authors contended – and some of the criticisms leveled at the study after its release, especially by the United States.

The lecture was followed by a lengthy discussion period, attended by some 40 NGO and government legal advisers, scholars and students.