Groundwater Dependent Ecosystems (GDEs) frequently exhibit rich biological diversity and can provide enormous economic wealth. In recent years, GDEs in many industrialized countries have shown signs of serious degradation, primarily the result of groundwater abstraction and pollution. Many such systems, including a number of well documented cases in Eastern Europe, are no longer sustainable. As a consequence, the conservation and sustainable management of GDEs has emerged as one of the most urgent environmental research priorities of our time. A large percentage of the world’s population lives in cities and either depends on, or is affected in some way, by groundwater. Moreover, groundwater has become a very important and complex issue that attracts the interest of many diverse stakeholders. Many problems related to groundwater and ecosystems are shared by countries throughout the world and there is growing recognition that much can be gained by co-operation on an international scale. This is no time to be complacent and it is critical that key problems be identified, that the potential consequences of these problems be understood, and that the development of solutions begins urgently. Important data gaps must be recognized and filled without delay