After 9/11 and in particular after the Madrid attacks in 2004, EU legislative activity related to internal security has been rapidly accelerating. Since then, the European Union has been very active in building a European security regime that is far from being limited to counterterrorism but spans the range of security policies. This work analyses the legal and political instruments developed by the European Union which are aimed at improving law enforcement cooperation between the Member States. Supported by a critical assessment of the European data protection framework, it is argued that the development of extensive security regimes is not sufficiently counterbalanced by a strong fundamental rights regime or effective institutional safeguards, leading to an imbalance between security and liberty in the European Union. Providing a critical argument based on a careful legal analysis, this work should be helpful to academics and others interested in the discourse on EU security governance and civil liberties since 9/11.