In recent years, transboundary aquifers have received growing attention in numerous policy-making and negotiating circles. This development suggests an evolution in customary international law for transboundary aquifers. This article examines recent international arrangements and pronouncements related to the assessment, use, allocation and protection of transboundary groundwater resources and identifies the legal trends emerging from these instruments. The article also considers gaps and shortcomings in the emerging regulatory regime and offers recommendations for the further development of the law.