In recent decades, the use of groundwater has been increasing around the world, in response to the rising demands for drinking water supplies and food production for a growing global population. In many places this has led to overexploitation of local and regional aquifers owing to poorly informed management of groundwater resources. A comprehensive understanding of the resource, its characteristics and its uses are the basis for science-based and informed decision making and planning. This is of special importance in the case of transboundary aquifers (TBAs) that are shared between two or more countries. UNESCO is playing a leadership role in improving the governance of TBAs. UNESCO's global multi-partner initiative the Internationally Shared Aquifer Resources Management (ISARM) program is conducting an inventory of TBAs globally which will guide Member States towards the sustainable management of shared groundwater resources. In 2012 the Global Environment Facility (GEF) endorsed the implementation of the "Transboundary Waters Assessment Programme" (TWAP). The objective of TWAP is to apply indicator-based assessment methodologies to conduct a global assessment of TBAs. The UNESCO IHP project "Groundwater Resources Governance in Transboundary Aquifers" (GGRETA), funded by the Swiss Development Cooperation is carrying out a more detailed assessment than the TWAP including a larger set of indicators and using spatially and temporally distributed data. The GGRETA project includes three case studies: the Trifinio aquifer in Central America, the pre-Tashkent aquifer in central Asia and the Stampriet aquifer in southern Africa. This presentation concentrates on the Stampriet aquifer system that straddles the border between Botswana, Namibia and South Africa. The Stampriet case study has been selected because implementation is proceeding relatively smoothly and quickly. The Stampriet system is an important strategic resource for the three countries. In Namibia and Botswana the aquifer is the main source of water supply for agricultural development and urban centers in the region, while in South Africa the aquifer supplies stock ranches and a game reserve. The GGRETA Project, which is taking place over the period 2013-2015, adopts a 2 phase approach to multi-country resource management. The first phase of the project builds recognition of the shared nature of the resource, and mutual trust through joint fact finding and science based analysis and diagnostics. This begins with collection and processing of hydrogeological, socio-economic, environmental, legal and institutional data at the national level using a standardized set of variables developed by the International Groundwater Resources Assessment Center (IGRAC). In the Stampriet region this stage of the project has been completed successfully. The next stage of the project involves harmonization of national data in which Botswana Namibia and South Africa will agree on harmonized classifications, reference systems, language, formats and derive indicators from the variables. The variables and indicators include a gender based component, the first time a systematic gender based analysis of the transboundary aquifer has been attempted. Harmonized data on the transboundary aquifer will be fed into an aquifer information management system developed by IGRAC. The harmonized data will provide the basis for an integrated assessment of the Stampriet transboundary aquifer in the first part of 2015. This assessment will assist the case study countries to set priorities for further collaborative work on the aquifer and to reach consensus on the scope and content of multicountry consultation mechanism aimed at improving the sustainable management of the aquifer. This phase of the project includes training for national representatives in international law applied to transboundary aquifers and methodology for improving inter country cooperation. This methodology has been developed in the framework of UNESCO's Potential Conflict Cooperation Potential (PCCP) program. A major success of the project to date has been the creation of the national technical teams which have carried out national data collection and are now undertaking data harmonization prior to the joint aquifer assessment. The project has also increased cross-country dialogue, coordination and collaboration. The project has benefited from strong support from national water management agencies. Information gaps have provided the biggest challenge in the project to date. For example, there are few long period time series of borehole data or water abstractions. These information gaps are being addressed by further data collection efforts and the use of surrogate measures, such as stock numbers as an indicator for water abstractions. Thanks to the efforts of the national technical teams well supported by national administrations there are good prospects that the project will produce a comprehensive aquifer assessment that will provide a platform for the development of new mechanisms for multicountry cooperation and improved sustainable aquifer management.