Joint ventures are a widely used form of interfirm collaboration, which possess some characteristics of both mergers, in which two or more firms come permanently under common control and cease to be distinct, and agreements, which impact the competitive behaviour of firms that remain independent of each other. Due to their hybrid economic nature, it has always been controversial what types of joint ventures should be regarded as mergers under competition law, and how the fact that the parent firms remain independent outside the cooperation should be incorporated into traditional merger analysis. In the EU, only full-function joint ventures are treated as mergers under the Merger Regulation. However, unlike amalgamations and acquisitions, these joint ventures are subject to an additional analysis under Article 101 TFEU to the extent that they lead to the coordination of the competitive behaviour of the parents. This thesis aims to critically analyse this approach to the substantive appraisal of joint ventures under the EU merger control regime, based on its consistency with the general rules of EU competition law and with relevant economic theories, in a comparative perspective with the US competition law regime. In this regard, the thesis shows that (i) the full-functionality criterion assuming that full-function joint ventures are autonomous from an operational viewpoint has some negative implications for the competition analysis of joint ventures; and (ii) the fact that the parent firms retain activities in the joint venture’s market, or in other markets, does not seem to be properly incorporated into the analysis under the Merger Regulation. This thesis seeks to propose some alternatives and solutions with respect to these problematic issues and, thereby, contribute to the body of knowledge and debate about joint ventures which have been, and will be, one of the most intricate and controversial topics of competition law.