The protection of fundamental rights and procedural guarantees in administrative investigations conducted by the European Anti-Fraud Office (OLAF) has constantly evolved since its creation in 1999. First, the catalogue of procedural rights and guarantees embedded in the successive regulations governing the conduct of OLAF’s investigations was significantly expanded. Second, the existing mechanisms to ensure their protection were reinforced by the creation of the new function of the Controller of procedural guarantees and a new complaints mechanism. As a result, the legal framework under which OLAF operates at present is significantly more protective of fundamental rights and procedural guarantees than it was 25 years ago. This article sheds light on these two key developments: the progressive codification of fundamental rights and procedural guarantees applicable to OLAF’s investigations and the reinforcement of their protection by the creation of the new Controller and the new complaints mechanism.