No other markets have likely ever been as closely part of our everyday lives as digital markets have. Digital markets can be described as a place for innovation where new products, services, and companies pop up on a nearly daily basis. These markets can at the same time be described as highly concentrated, in the hands of a few, who control the market, and make the rules on the market. It is without a question that digital markets are full of possibilities for innovative companies and therefore it is of such importance that these markets are protected and kept as fair, competitive, and transparent as possible. There have been concerns whether the existing competition law is the best tool to be used on digital markets and whether the often-lenghty process of Article 102 TFEU can, or at least effectively can, control these gatekeepers controlling digital markets. The purpose of the thesis is therefore to analyse the existing and new theories of harm when it comes to Article 102 TFEU and whether they could be useful in the Commissions' ongoing investigations against these gatekeepers. As existing theories of harm margin squeeze, unfair contractual terms and conditions as well as tying will be analysed and their applicability in the digital markets. While some conducts might fall under the existing theories of harm others won't and thereof it is worth looking into new theories of harm such as self-preferencing, forced free riding, and privacy policy tying, in order to determine whether they could be used by the Commission.  While the application of Article 102 TFEU is of great importance, the Commission has also drafted a proposal called Digital Markets Act which would be an ex-ante tool to be used alongside EU competition law and which could help address the problems the current competition law isn't at least effectively addressing. The Digital Markets Act is based on certain undertakings when fulfilling the criterium, being defined as gatekeepers, and when defined as such, the obligations of the Digital Markets Act become applicable. This would offer the Commission a tool that is likely more efficient and legally certain than the application of Article 102 TFEU. Since the existing competition law hasn't been able to prevent digital markets becoming even more and more concentrated, the Digital Markets Act might be exactly what is needed in order to secure the internal market remaining competitive, fair, contestable, and transparent.