Fighting cartels is a major priority of EU competition policy. Acting in concert with national com¬petition authorities in the EU, the European Commission (EC) has made considerable efforts to promote competitiveness by detecting and punishing cartels. Many commentators argue that the introduction of the EC leniency programme (LP) in 1996 is likely a key enabler. Given the substantial increase in both the number of detected cartels and the average fine per cartel member, it comes as no surprise that a growing number of convicted firms have con¬sidered filing an appeal against EC decisions. This paper presents the key results of an empirical ZEW study on the determinants of self-reporting under the European LP and discusses the characteristics of firms filing an appeal, as well as the factors that determine their success in terms of fine reduction.