The Guarani Aquifer System is an important transboundary groundwater reservoir located in central-east part of South America and shared by Argentina, Brazil, Paraguay and Uruguay. Groundwater use in the region has increased very fast since the 1970s.

During the last decades, the four countries have developed laws, institutional frameworks and regulations related to water resources management in general, but gaps in groundwater issues still exist. After previous scientific studies showing the importance of the aquifer, these four countries have jointly prepared and implemented the Project on the Protection and Sustainable Development of the Guarani Aquifer System (PSAG, 2003–2009), with the support of the Organization of the American States and the World Bank, and funded by the Global Environment Facility and other international organizations. The main project objective was to support countries in establishing a management framework for the Guarani Aquifer. As a result of the scientific knowledge developments, the project appropriation process by country representatives and a structured project execution process carried out by the project team, a Strategic Action Plan (SAP) was developed. Additionally, the countries signed an Agreement on the Guarani Aquifer (2010), now under national parliament approval in Paraguay (Argentina Brazil and Uruguay have already approved it).

The objective of this chapter is to discuss the Guarani Aquifer Governance perspectives in practical terms, correlating the existence of applicable management instruments and the responsibilities of different institutional levels involved in groundwater regulation in the countries. Four institutional levels are distinguished: a) Regional or global; b) national; c) state or province; and d) municipal or local. Currently, the main management responsibilities are at the national level in the unitary countries and at state or province level in the federal countries. Besides that, according all knowledge developed by the Guarani Aquifer Project the weakest and most crucial level to foster groundwater governance is the local or municipal level, because it is at this level that all contamination and overexploitation problems of the aquifer really occur. Many expectations are supposed to be resolved after the Guarani Aquifer Agreement’s enforcement; one of them is how regional and national level can effectively support the local level, a critical dimension to mitigate impacts and develop protection strategies to the Guarani Aquifer.