The core of this thesis studies adverse effects of cartels on the functioning of the internal market of the European Union (EU). To understand the topic in its entirety, the paper firstly apprehends the nature of cartels, their definition, features, and modus operandi. By focusing explicitly on cartel regulation as a distinctive and significant part of competition law, the succeeding section presents the European Commission and national competition authorities, as the main enforcement bodies of the cartel prohibition, as well as the leniency programme and whistleblower-tool, innovative mechanisms employed to fight cartels. Once the enforcement mechanisms have been examined, the paper turns to adverse effects of such prohibited practices on the internal market, including on consumers and other businesses, namely in terms of production, prices, quality, market subdivision and competition. the paper turns. Cases presented should indicate a variety of the adverse effects that cartels have on the internal market of the EU. Additionally, towards the end, the paper provides an overview of Articles 34 TFEU, which prohibits the imposition of quantitative restrictions or equivalent measures on intra-EU imports, and Article 101 TFEU, which, in short, prohibits cartels. The purpose of this comparison is to highlight the similarities and differences between the two articles.