The Ramsar Convention has long recognized the critical need to understand the inter-relationships between water and wetlands – the preambular text of the Convention itself recognizes “the fundamental ecological functions of wetlands as regulators of water regimes and as habitats supporting a characteristic flora and fauna.” Over the years the Convention has developed and adopted an increasing suite of guidance for its Contracting Parties concerning different aspects of managing water for wetlands and managing wetlands for water, because to secure the conservation and wise use of wetlands it is essential that they be managed in the wider context of basin-scale water resource management. Much of this guidance to date has focused on aspects of managing surface waters in relation to wetlands, and this is provided in this 4th edition of the Ramsar ‘toolkit’ in Handbooks 8, 9 and 10. It is also critical, however, that both wetland and water managers understand the relationships between wetlands, surface water, and underground water – yet the roles and importance of such groundwater systems are much less well studied and understood. This Handbook has been designed as a contribution to filling this gap by providing guidance both to those responsible for managing water resources and to wetland managers.