The three papers reprinted in chapters 3 to 5 of this cumulative dissertation thesis investigate the effectiveness of European competition law and whether its main objective – to ensure a fair competition between companies – can be reached. Yet, competition is expected to maximize welfare as it forces companies to reduce prices or to improve their quality or service. However, competition can be distorted if companies agree with each other to fix prices, or if a company with a dominant market position uses its dominance to distort competition. Another possible threat to competition might come to the fore if a merger among large companies reduces competition or if governmental support to a specific company leads to an advantage over its competitors. Therefore, in order to ensure a fair competition among companies governments normally do not intervene in the process but set only the framework in which businesses operate. European competition policy as part of this regulatory policy includes all measures aimed at preserving competition and comprises four main pillars: Cartel law, Antitrust law, Mergers and Acquisitions and State Aid. The papers of this thesis focus on two of these four areas: Cartels and State Aid.