In approaching the writing of Intervention in the International Court of Justice, the author soon reached two conclusions. The first was that in order to understand the attitude of the Court today in applying the provisions of Articles 62 and 63 of the Statute, considerable importance attaches to their legislative history. In the Barcelona Traction (Preliminary Objections) case the Court referred to the `drafting records' of certain provisions of the Rules of Court under consideration there. The second conclusion was that examination of the decisions of the Court in intervention proceedings incidentally and in isolation from the case as a whole could put the material out of focus. The Court's position in a case of intervention can only be fully assessed in the context of the proceedings as a whole, when the real influence of the intervention phase on the final decision comes into the open. In addition, a new dimension, that of modern diplomacy, could be added to an understanding of the difficulties posed - for the Court, for individual States and for their legal advisers - by the institution of intervention.The book is based on a series of courses given by the author as Belle van Zuylen Professor in the Humanities at the University of Utrecht in 1986--1987. Nicaragua's intervention in the `Land, Island and Maritime Frontier Dispute' case between El Salvador and Honduras before a Chamber of the International Court led to a complete reexamination of the whole work, and to this current publication.