The Statute and Rules of the International Court of Justice are based on the premise that there are two parties in contentious cases which agree to resolve their dispute through bilateral adjudication. The intervention procedure is an exception to the bilateral nature of adjudication and broadens dispute settlement in cases where the direct interests or concerns of another state are engaged. Thus Article 63 of the Statute of the Court permits intervention in disputes in which the interpretation of a convention is at issue and to which the intervening state is a party. However, the procedure had not been unreservedly accepted by the Court until New Zealand intervened in Whaling in the Antarctic (Australia v Japan, New Zealand Intervening). This article discusses the past experience with the intervention procedure under Article 63 of the Statute, the scope and nature of the procedure, the extent of discretion on the part of the Court to admit an intervention under Article 63, the status of an intervening state, and the degree to which an intervention may affect the equality of the parties. It concludes by addressing the possibility that the Court’s decision in Whaling in the Antarctic will lead to a resurgence in recourse to the Article 63 procedure, potentially disturbing the contentious nature of the adjudicative process and threatening bilateralism.