The International Court of Justice (ICJ), often referred to as the World Court, is the principal judicial organ of the United Nations. Established in 1945 by the UN Charter, it succeeded the Permanent Court of International Justice (PCIJ) and is based at the Peace Palace in The Hague, Netherlands. Its primary functions are to settle legal disputes submitted to it by states and to give advisory opinions on legal questions referred to it by authorized UN organs and specialized agencies.

Composition of the ICJ:

The ICJ is composed of 15 judges, who are elected for terms of nine years. These judges are elected by both the United Nations General Assembly and the Security Council, voting independently but simultaneously. To be elected, a candidate must obtain an absolute majority of votes in both bodies. The elections are staggered, with five judges elected every three years to ensure continuity.

Key aspects of its composition include:

1. Nationality: No two judges may be nationals of the same state. This provision ensures a broad representation of the world's legal systems and cultures, preventing the Court from being dominated by any single national perspective.
2. Qualifications: Judges must be persons of high moral character, who possess the qualifications required in their respective countries for appointment to the highest judicial offices, or are jurisconsults of recognized competence in international law. They are expected to act as independent judges, not as representatives of their national governments.
3. Geographical Representation: The Statute of the ICJ mandates that the Court's composition should reflect the main forms of civilization and the principal legal systems of the world. This is achieved through an informal understanding among UN members to ensure a balanced geographical distribution across the five permanent members of the Security Council and other regions.
4. Presidency: The Court elects its President and Vice-President for three-year terms. The President presides over all meetings of the Court, directs its work, and has a casting vote in the event of a tie.
5. Ad Hoc Judges: If a state party to a case does not have a judge of its nationality on the bench, it may appoint an ad hoc judge (also known as a judge ad hoc) for that specific case. This ensures equality between the parties and maintains confidence in the impartiality of the Court.

Voluntary Jurisdiction of the ICJ:

The jurisdiction of the ICJ is fundamentally voluntary, meaning that states cannot be compelled to appear before the Court unless they have consented to its jurisdiction. This principle is a cornerstone of international law, reflecting state sovereignty. There are several ways states can express this consent:

1. Special Agreement (Compromis): Two or more states can agree to submit a specific dispute to the Court by concluding a special agreement. This is a clear and explicit form of consent for a particular case.
2. Treaty Provisions (Compromissory Clauses): Many bilateral and multilateral treaties contain clauses (known as compromissory clauses) where states agree in advance to submit any future disputes arising from the interpretation or application of that treaty to the ICJ. By ratifying such treaties, states implicitly accept the Court's jurisdiction for those specific types of disputes.
3. Optional Clause Declaration (Compulsory Jurisdiction): Article 36(2) of the ICJ Statute, known as the 'Optional Clause,' allows states to make a unilateral declaration recognizing as compulsory the jurisdiction of the Court in all legal disputes concerning certain categories of international law, in relation to any other state accepting the same obligation. This is often referred to as 'compulsory jurisdiction,' but it is still voluntary in the sense that states choose to make this declaration. These declarations can be made unconditionally or with reservations (e.g., excluding certain types of disputes or for a specific period).
4. Forum Prorogatum: If a state has not previously accepted the Court's jurisdiction, it may do so ad hoc for a particular case after proceedings have been instituted by another state. This can happen if the respondent state takes steps that indicate its acceptance of the Court's jurisdiction, such as filing a counter-memorial or appearing in court without explicitly challenging jurisdiction.

Despite the term 'compulsory jurisdiction' for the Optional Clause, it is crucial to remember that the ICJ's power to hear a case ultimately stems from the consent of the states involved. This voluntary nature is both a strength, as it respects state sovereignty, and a limitation, as it can prevent the Court from adjudicating important international disputes if one party withholds consent.