»Breach of Res Judicata under Union Law«: Res judicata is necessary to give final effect to court decisions. For this reason, res judicata is also indispensable for Union law. Nevertheless, it can also be an obstacle to the uniform enforcement of Union law. Especially when Member State courts misapply Union law, the question arises whether the court decision can continue to stand. This thesis deals with the clarification of the question under which conditions the uniform enforcement of Union law is to be given priority over res judicata.