Since States alone have capacity to appear before the Court, public (governmental) international organizations cannot as such be parties to any case before it. A special procedure, the advisory procedure, is, however, available to such organizations and to them alone.
Though based on contentious proceedings, the procedure in advisory proceedings has distinctive features resulting from the special nature and purpose of the advisory function.
Advisory proceedings begin with the filing of a written request for an advisory opinion addressed to the Registrar by the United Nations Secretary-General or the director or secretary-general of the entity requesting the opinion. In urgent cases the Court may do whatever is necessary to speed up the proceedings. In order that it may be fully informed on the question submitted to it, the Court is empowered to hold written and oral proceedings.
A few days after the filing of the request, the Court draws up a list of those States and international organizations likely to be able to furnish information on the question before the Court. In general, the States listed are the member States of the organization requesting the opinion, while sometimes the other States to which the Court is open in contentious proceedings are also included. As a rule, organizations and States authorized to participate in the proceedings may submit written statements, followed, if the Court considers it necessary, by written comments on these statements. These written statements are generally made available to the public at the beginning of the oral proceedings, if the Court considers that such proceedings should take place.
Contrary to judgments, and except in rare cases where it is stipulated beforehand that they shall have binding effect (for example, as in the Convention on the Privileges and Immunities of the United Nations, in the Convention on the Privileges and Immunities of the specialized agencies of the United Nations, and in the Headquarters Agreement between the United Nations and the United States of America), the Court's advisory opinions have no binding effect. The requesting organ, agency or organization remains free to decide, by any means open to it, what effect to give to these opinions.
Although without binding effect, the advisory opinions of the Court nevertheless carry great legal weight and moral authority. They are often an instrument of preventive diplomacy and have peace-keeping virtues. Advisory opinions also, in their way, contribute to the elucidation and development of international law and thereby to the strengthening of peaceful relations between States.