Globally, the vast majority of countries share aquifers with their neighbours. Political, socio-economical, cultural and other differences among the countries make the assessment and the management of internationally shared aquifers more difficult, comparing with national ones. Insufficient knowledge of transboundary (parts of) aquifers and a lack of coordinated management can lead to unwanted changes in groundwater flows, levels, volumes and dissolved substances. Therefore, an appropriate assessment of transboundary aquifers (TBAs) is required to prevent or mitigate groundwater problems and to improve the overall benefit from groundwater. This paper discusses the contours of a general assessment methodology. This methodology is needed to guide a user through the assessment steps, pointing out the specifics of various aquifer issues in the international context. Besides, it should alleviate comparison among TBAs and encourage the sharing of experiences gained elsewhere.