The research in this doctoral thesis examines the protection of personal data in two relationships: between judicial and law enforcement authorities of the EU Member States (including Europol and Eurojust) on the one hand and the US on the other hand. The book provides answers to the central question as to whether the EU complies with her own standards of data protection in these internal relations, as well as in the transatlantic cooperation in criminal matters. The new framework decision on data protection in criminal matters that entered into force in early 2009 is a significant element in this study, alongside the agreement concluded between the EU and the US on the mutual assistance in criminal matters of which the entry into force is equally planned for 2009. The book also reflects on the policy proposals of the EU on justice and home affairs for the period of 2010-2014.