In 2008, the United Nations' International Law Commission (ILC) completed work on its Draft Articles on the Law of Transboundary Aquifers (ILC 2008). The United Nations General Assembly adopted a resolution later that year “taking note” of the articles and “encouraging” member states to consider the articles in making arrangements for their transboundary aquifers. Four years earlier, the International Law Association completed its own codification of this same body of law. This article explores the nature of customary international law, its application to internationally shared groundwater and the relative merits of these two codifications of the customary international law of groundwater.