This thesis analyses the Europeanization of national environmental agencies by assessing the impact of the&nbsp; European Environment Agency (EEA) and its main environmental information and observation network, the&nbsp; Eionet, on three of its member countries, namely Germany, France and Britain.The EEA began its work in 1994. It established the Eionet to institutionalize cooperation with member countries&nbsp; from which it obtains environmental data required for its work. This thesis assesses the German Umweltbundesamt (UBA), French Agence de l’Environnement at de la Maîtrise de l’Energie (ADEME) and Institut Français de l’Environnement (Ifen) as well as the Environment Agency (EA) of England and Wales. The different national arrangements for Eionet participation are explained and the question of whether the creation of the EEA and national participation in the Eionet had a significant impact on the national environmental administrations in the three case countries is scrutinised. It is argued that all national environmental agencies assessed in this thesis have been affected by Europeanization, although to different degrees. This thesis draws heavily on historical institutionalism and Europeanization theories when 'testing' three hypotheses. Unpublished new empirical findings are also presented.This thesis argues that the EEA‘s impact on its member countries has, overall, remained very limited which explains the continued divergence between national environmental agencies. These findings are in line with historical institutionalist explanations. The only exception is the French Ifen which was set up as an independent agency in direct response to the creation of the EEA. As explained in the thesis, the French exceptionalism was, however, short-lived and largely driven by domestic (rather than EU-level) factors. This thesis provides new empirical material and analytical insights into the cooperation of national environment agencies and the EEA within the network of Heads of European Environment Protection Agencies (EPA network)