The diploma thesis addresses the issue of the effect of the introduction of leniency program and its reforms on the European competition policy and above all of its effect on investigation of forbidden cartel agreements. Leniency program is currently one of the main tools being implemented in fighting cartels. It works by providing immunity to the first company that provides relevant proof of the cartels existence. This work has three main goals. Firstly it aims to analyse the effectiveness of the leniency program and its reforms when it comes to uncovering cartels. The second goal is to unveil the European Commissions strategy concerning granting leniency and its development over the time. The last goal is to provide a quality analysis of ...