The ascending economic and political influence of the internet platforms of the 21st century has sparked a debate about the adequate regulation of these “tech titans”. At the heart of this discussion is competition law – the field of law that was created to tame dominant companies. But when does a company truly hold such a “dominant position”? The definition of this fundamental competition law term faces numerous challenges when applied to digital platforms, from zero-price markets to the multi-sidedness of business models. This book dismantles the term into its components and shows where the methodology needs to adapt to the digital economy. In doing so, it considers the legal regimes of Germany, the EU and the US, as well as findings from legal economics.