There is a growing international consensus that standard competition law is inadequate for addressing the panoply of competition problems arising in digital platform markets. Alongside a proposal for ex ante regulation in this arena, the European Commission is considering the introduction of a ‘New Competition Tool’ which is broadly modelled on the UK Market Investigation instrument. This chapter abstracts from the specifics of the EU situation and considers the pros and cons of market investigations in the context of the UK regime. It concludes that the tool is a valuable addition to the standard competition law toolkit, and that this is likely to be true also at EU level, both for digital platforms and more widely. However, because the tool is potentially so powerful and flexible, it merits strong procedural checks and balances, to guard against confirmation bias or politicisation. The tool also has important limitations and thus should not be viewed as a full solution to the issues raised by digital platforms, but rather as a valuable complementary tool alongside new ex ante regulation. Interoperability is discussed as one area where the tools may act in a complementary way.