It is now widely accepted that groundwater, though not as visible as surface water, is ubiquitous in the global landmass, is contained in the pore spaces of rock formations (aquifers), and its science, hydrogeology, has rapidly developed over the last 35 years. This science has contributed to the well-being and development of the human population in all parts of the globe (Burke and Moench, 2000). While aquifer systems, due to their partial isolation from surface impacts, on the whole contain excellent quality water, there has been poor resource management because economic externalities have been ignored. In many countries, aquifers have been fully evaluated and extensively used for municipal and other demands, as a common good. Aquifer resources represent a substantial hidden global capital, which if abused, leads to future economic costs and environmental-social conflicts. As a result, the beneficial use of groundwater should be more particularly subjected to socio-economic, institutional, legal, cultural, ethical and policy considerations than surface water. Its national sustainable management seems to be hampered by weak social and institutional capacity, and poor legal and policy frameworks. In a transboundary context, this can be even further amplified because of contrasting levels of knowledge, capacities and institutional frameworks on either side of many international boundaries. UNESCO and its partners have established a global programme to address the long-term sustainability for groundwater resources, through a series of actions, including ISARM and PCCP. In this chapter, which is an overview designed to provide the NATO ASI with the necessary background, these actions, and the progress achieved over the past decade will be described so that participants can gain the necessary background to take the agenda forward. Some lessons learned from the inventory process of the Americas are presented, followed by a review of the linkage between transboundary aquifers and globalization, and a presentation of groundwater dependent ecosystems and governance issues. The chapter concludes with an outlook of the UNESCO programme on transboundary water resources encompassed in two very successful initiatives, the PCCP and ISARM
