As a glance at his bibliography shows, Dr. Sipkov, as well as a law librarian, was a publicist in the original sense of the term — a scholar of public, in particular public international, law. It is therefore fitting that our comment deals with a basic issue that arises at the intersection of international law and librarianship. We would like to evoke the problem of defining and then finding the “material sources” of customary international law. That means understanding, then locating, the concrete, usually documentary, materials that must be used to confirm the existence of a state practice. The consistent, generalized, though not necessarily universal, practice of states forms the objective element of a customary rule. This basic problem of international legal research is not new, but as we approach the fiftieth anniversary of the United Nations International Law Commission, it makes sense to examine it again. Developments over the last half century should help in understanding how we try currently to solve the difficulties and will do so in the future.