Codification of international law may be pursued by various methods. This contribution analyzes advantages and disadvantages of codification conventions. Treaties defined as codification conventions express customary rules in a revised form. They often contain provisions that cover matters not previously regulated or that supplement or specify existing rules. Whether a treaty displaces a customary rule in the relations between the states that are parties generally is a matter of interpretation of the relevant treaty. The limited number of ratifications of codification conventions that have entered into force raises problems concerning the relations between these conventions and customary rules. The present contribution also examines the consideration by the International Court of Justice of the relations between codification conventions and customary rules. In many cases, the Court has not provided reasons for concluding that the content of the customary rule corresponds to that of a provision in the codification convention. In recent years, the General Assembly has often chosen not to promote a codification convention, but there is no good reason for abandoning recourse to an instrument that has significantly contributed to the development of international law in several areas.