The establishment of the EPPO is an unprecedented milestone in the area of freedom, security and justice in the EU. The EPPO is not a body for mere coordination or even collaboration between judicial authorities, but a fully-fledged, supranational, procedural actor, separate from the respective national prosecutors' offices, with its own structure, bodies and material resources, which acts independently of both the EU itself and its Member States in criminal proceedings. The EPPO is therefore facing a major challenge, which is the need to combine in the same criminal procedure the European Regulation and the respective national procedural laws of the participating Member States. Investigation and prosecution will be carried out by the EPPO in accordance with the Regulation and, as regards matters not covered by it, in accordance with national procedural law; the trial, however, will take place before the competent national courts in accordance with the corresponding national procedural law. Recital 15 of the EPPO Regulation excludes any intention to change the organisation of criminal investigation in the participating Member States. However, the fact is that participating Member States have had to find their own way to "insert" the EPPO into their respective existing procedural systems. Although the EPPO Regulation is, as such, directly applicable in the participating Member States, it has determined, to a greater or lesser extent, a need for substantial legal reforms in national systems. The implementation of the EPPO constituted a particularly substantial challenge for those Member States where an investigating judge has the leading role in the pre-trial phase of criminal proceedings, as is the case in Spain. In none of its precepts does, the Regulation excludes the intervention of an investigating judge. Nevertheless, it was clear that it could imply, at the very least, a redefinition of the perimeter of the investigating judge's powers, if not the establishment of a new and specific procedural framework for offences where the EPPO exercises its competence. The latter is what has happened in Spain. This paper describes the pathway followed in Spain to implement the EPPO and the different solutions adopted, as challenges have emerged. Only experience will show whether those legal solutions are the right ones. Further modifications cannot be excluded in the future, as experience might show potential dysfunctions.