The interpretation and application of different provisions of the Concierto Económico (Economic Agreement) on the coordination, fiscal harmonization and collaboration between the Basque Country and central government are questions that are always controversial. This work is based on the study of the nature of the system of Agreements: its origin, historical roots, developments and how they fit into the legal and political framework currently in force in Spain, without losing sight of the European Community background. Second, as an essential aspect of the study, it examines the network of harmonizing rules that are part of the text of the Agreement itself. Finally, the study places particular emphasis on the rule referring to the overall effective tax burden applicable to the Basque Country.