The paper analyses the role and importance of the principle of reciprocity in the optional clause system of the International Court of Justice. After a short description of the Statute provisions on reciprocity of the two International Courts the author deals with the stipulation of reciprocity in declarations accepting the compulsory jurisdiction of the Court. The main part of the paper is devoted to the legal practice of the two International Courts on the matters of reciprocity. As a conclusion the author says that, by virtue of the principle of reciprocity, reservations to the acceptances of compulsory jurisdiction tend, in practice, to make their effect felt more often than not, precisely against the State or States making a reservation.