The assessment of the Stampriet Transboundary Aquifer System (STAS) was undertaken based on a multi-disciplinary methodology developed by the UNESCO International Hydrological Programme (IHP) and the International Groundwater Resources Centre (IGRAC) that includes the collection and processing of national data (hydrogeological, socio-economic and environmental, gender, legal and institutional), and the harmonization of data across all three countries to enable a joint assessment of the transboundary resource. Activities were done within the framework of the Governance of Groundwater Resources in Transboundary Aquifers (GGRETA) project, executed by UNESCO-IHP, in close cooperation with IGRAC, and funded by the Swiss Agency for Development and Cooperation (SDC). The assessment has been carried out by a team familiar to the area and composed of professionals of Botswana, Namibia and South Africa. Apart from collecting and studying relevant literature for assessment and diagnostics, the team has spent much attention to compiling basic data and to GIS mapping. In total, more than 40 thematic maps were prepared and have been uploaded to the Information Management System (IMS) developed by the project.