This book conducts the first ever comprehensive study of the ICRC's interpretations and law-ascertainments. It analyses in detail their impact on the development of international humanitarian law and international law in general as well as the reasons for their impact. This analysis involves the discussion of the ICRC's authority. Is it legal or just factual authority? The analysis also illuminates the direction that IHL - and international law in general - develops. This insight sheds light on the question of the current type of international law, i.e., what international law is and who makes it.