There is no honour among thieves. This aphorism concisely expresses why the leniency programmes in competition policy have become one of the most effective instruments in the fight against the cartels. In this work we describe the dissemination, evolution, and effects of the aforementioned programmes in the two decades since its implementation around the world, paying special attention to what is being done at the European Union level and in Spain. The empirical regularities obtained from the descriptive analysis of leniency decisions adopted by the European Commission and by the Spanish Competition Authority provide relevant information about the effectiveness of their corresponding programmes, as well as information about the underlying reasons why companies, in this context, submit applications for sanction exemption or reductions in the penalty amount. We conclude that still there is scope to increase substantially the dissemination and implementation of the leniency programme in Spain, and if the reforms are handled correctly, the programme is set to catch up and to be the main source of detecting and sanctioning of Spanish cartels in the next decade