It is a particular pleasure to contribute an essay on the sources of international law to this Festschrift for Sir Robert Jennings in recognition, not only of his distinguished contributions to this topic both as an academic and as a judge, but also of personal acts of kindness.

Every case in the World Court involves the sources of international law in some form, because every case requires an investigation of substantive or adjective rules whose existence and legal effect ultimately depends on their having been created by one of the means recognized as apt for this purpose. Even if one eliminates cases where there was no issue as to the status of the rule, but only as to its content, in the fifty years since the International Court of Justice was established it has had very many occasions to make rulings pertinent to our topic. In the space available here, any review of its contribution is inevitably summary and impressionistic.

The ICJ has not attempted to elaborate a theory of the sources of international law or attempted to catalogue them. This is hardly surprising: its function is to decide the particular disputes before it, not to elaborate general theories or to decide questions that are not in issue.