First introduced in the United States in 1993, leniency programmes have, according to the OECD Competition Trends Report 2020, ‘grown rapidly… reaching 89 jurisdictions by 2017’. They have been declared a great success at detecting cartels. This foreword will walk the reader through a brief overview of the development of leniency policy in the European Union from 1996 to date. I do not intend to provide an analysis of US developments save to note it has recently enacted legislation to protect whistleblowers, recognising the value of their input. A browse through the Commission’s own records of cartel decisions illustrates graphically its policies in action. Cases burgeoned to such an extent that concerns were expressed that the Commission had become a victim of its own success and that the staff of DG Competition would not be able to handle the volume of cases. A few cases where whistleblowers uncovered a cartel will be mentioned in the latter part of this foreword. In the discussion of the context of damages actions, I will thereafter provide a review of some of the cases reviewed by the Court (General Court, European Court of Justice, High Court of England and Wales). The aim is to contextualise the leniency tool, as one of many in the European Commission’s toolbox for enforcement against what is considered to be the most egregious of competition law infringements.