This book offers a detailed overview of the rules regarding criminal investigations into financial-economic criminality in the EU's main legal systems. These rules have become fundamental to the effective protection of the Union's financial interests. It undertakes a comparative study of six national legislatures (Italy, Spain, France, Germany, Poland, the UK) which serve as paradigms of the different judicial systems existing in the Union, in order to offer a complete overview of the different approaches to financial-economic investigation in the EU. The work is further enriched with cross-sectional essays that deal with the more general issues, such as data-protection and the future of investigations in the view of the establishment of the European Public Prosecutor's Office (or EPPO). This provides a wider perspective on the themes considered. The book also examines trans-national issues, providing essential context to the EU's legislative instruments intended to protect the financial interests of the Union.