This is the first monograph on the concept of obligations erga omnes, an increasingly important notion in contemporary international law since its appearance in the International Court's judgment in the Barcelona Traction case. In 1970, The International Court of Justice identified a separate category of international obligations, which rather than being owed to a specific State on a bilateral basis, are owed to the international community as a whole, with all States having an interest in their protection and their being binding on all States irrespective of consent. Examples of such obligations given by the International Court are those prohibiting aggression and genocide, and those protecting from slavery and racial discrimination, in other words obligations erga omnes are those protecting and promoting basic values and common interests of states. This book is the first to examine the very concept of the obligations, rather than the corresponding rights of protection of obligations erga omnes. Ragazzi adopts a pragmatic method of legal analysis that identifies five common elements among the examples of obligations erga omnes given by the International Court. These five common elements are then discussed in relation to other candidates of obligations erga omnes which have emerged in State practice and international literature. Ragazzi's approach is extremely thorough and includes extensive bibliographical references in various languages.