This article discusses the theory and practice of leniency in antitrust enforcement, i.e. the granting of
immunity from penalties or the reduction of penalties for antitrust violations in exchange for cooperation
with the antitrust enforcement authorities. After a description of the practice of leniency in the United
States and in the European Union, and of its history, the article analyses the positive effects and the
possible negative effects of leniency on optimal antitrust enforcement, and the extent to which these effects
can be measured. Objections of principle and institutional problems that may constitute obstacles to the
introduction of leniency policies are discussed, as well as some further issues, namely the impact on the
effectiveness of leniency of criminal penalties on individuals, of follow-on private damages actions, and of
penalties in other jurisdictions, ’’Amnesty Plus’’, and positive financial rewards or bounties.
World Competition