The intention of this monograph is to contribute to a better understanding of the crucial role played by groundwater resources in the support of both the ecosystems and mankind. This volume does not claim to offer the full picture as far as world groundwater resources are concerned; indeed our knowledge of the world and its most important aquifer systems is far from complete. As such, in years to come, some of the information contained in this monograph will require updating and this through results of new studies and activities initiated and carried out by UNESCO.