This research study addresses questions of institutional creation and change and contributes to two bodies of literature, namely, institutional theory and European Union politics. The broad question it asks is why different political systems contain similar institutions. I am specifically concerned with institutional similarities between the European Union and its member states. To provide an answer, I have conducted two extensive case studies, on the European Court of Auditors and the Office of the European Ombudsman. Both are European Union bodies which belong to a specific class of governmental institutions, namely independent organizations responsible for public accountability. I examine the two cases comparatively, in order to evaluate the explanatory power of the theory of institutional isomorphism against alternative perspectives on the basis of qualitative data, mainly official documents and elite interviews. Isomorphic theory asserts that similarities among different systems can be explained by the exercise of homogenizing pressures by actors. I examine which of the three main forces proposed by the theory are necessary to explain institutional similarity. On the basis of the two case studies, I find that the collusion of coercive pressures from member state governments and tendencies of EU institutions to mimic member state practices is a necessary condition for the transfer of institutions to the European Union structure.