The study was first proposed by the open-ended Intergovernmental Group of Experts for the Protection of War Victims which in January 1995 adopted a series of recommendations aimed at enhancing respect for international humanitarian law, including a study on customary humanitarian law to be undertaken by the ICRC. In December of the same year, the 26th International Conference of the Red Cross and Red Crescent, which comprises all States, endorsed these recommendations by consensus and, thus, officially mandated the ICRC to prepare a report on customary rules of international humanitarian law applicable in international and non-international armed conflicts.

The study was published in 2005 and is the outcome of more than 8 years of research and consultations carried out under this mandate. The study collects and analyses State practice in order to assess which rules of humanitarian law reflect widespread and uniform State practice and legal opinion and can therefore be considered part of customary international law.

Because customary international law requires widespread practice, the study had to cover State practice from all over the world covering various armed conflicts. In order to do so, 47 national rapporteurs were commissioned to examine national sources of practice and 6 international rapporteurs were commissioned to examine international sources of practice.

The collected State practice determines the rules of customary international humanitarian law contained in the study. The assessment was carried out by the authors of the study, Jean-Marie Henckaerts and Louise Doswald-Beck, who enjoyed complete academic freedom in carrying out their work. The idea was that the study was a work of scholarship aimed at capturing the clearest possible "photograph" of customary humanitarian law as it stands today. However, the authors did not operate in isolation. The study was conducted under the guidance of a Steering Committee of 12 renowned international law professors and consultations were held throughout the genesis of the study with renowned governmental and academic experts from many States around the world.

The customary status of the rules set forth in Additional Protocol I has regularly been debated. Today, Protocol I enjoys widespread support as it has been ratified by more than 160 States, including nearly all NATO members. The study on customary law concludes that several principles set forth in Protocol I as customary because they are practised and supported extensively and uniformly, not just by States party to the Protocol but also by States not party to this treaty. On the other hand, the study identifies a number of rules contained in Protocol I on which disagreement has been expressed. Therefore, these rules have not been identified as rules of customary law in the study (for example, on the presumption of civilian status in case of doubt). Other rules have been formulated differently from Protocol I to reflect this disagreement (for example, the rule on installations containing dangerous forces).

The ICRC believes that the study presents an accurate photograph of the current state of customary humanitarian law. It will therefore duly take the outcome of this study into account in its daily work, while being aware that the formation of customary law is an ongoing process. The study should therefore also serve as a basis for discussion on the implementation, clarification and development of humanitarian law.