The key goal of UNECE second Assessment of Transboundary Rivers, Lakes and Groundwaters, currently prepared under the Convention on the Protection and Use of Transboundary Watercourses and International Lakes (Water Convention), is to facilitate informed decision-making on the management of shared water resources in Europe, Caucasus and Central Asia, provide the basis for continuous bilateral and multilateral cooperation under the Water Convention, and support all actors involved at the national, transboundary and regional levels. In the second Assessment, a holistic, integrated approach is applied, looking at both surface and groundwaters within each transboundary basin. Pressures on transboundary water resources are identified and their relative importance in the transboundary context evaluated. Legal, institutional and socioeconomic aspects are highlighted and cross-cutting themes that are a challenge for managing transboundary waters — such as predicted impacts of climate variability and change — are emphasized. The extent of transboundary cooperation (joint bodies, joint monitoring etc) currently in place and measures taken are also described. The assessment report will provide information on more than 140 transboundary rivers, more than 30 transboundary lakes and some 200 transboundary aquifers. The Assessment is prepared in close cooperation with national experts nominated by environment administrations of the participating countries. In this paper, the approach and some preliminary results of the second Assessment in South-Eastern Europe, Eastern and Northern Europe, and the Caucasus are described, with focus on transboundary aquifers.