Introduction This chapter will provide a detailed consideration of the market investigations scheme in the United Kingdom. This is a unique set of rules which have evolved in the UK to play a signi cant role alongside the ‘classic’ or traditional competition law prohibitions of the type exempli ed by the Chapter I and II prohibitions and Arts 101 and 102 TFEU. Although the provisions have been considerably modi ed by the Enterprise Act 2002 and the Enterprise and Regulatory Reform Act 2013, the market investigations scheme embodies the traditional UK approach to competition law. 1 Essentially, market investigations do not determine whether behaviour is legal or illegal; the process does not focus on the attribution of blame nor impose sanctions for past conduct. The broad purpose of the market investigation system is to determine whether the process of competition is working effectively in markets as a whole. This is re ected in the approach to remedies which generally focus on ensuring better, more effective competition in markets for the future in the interests of consumers. Accordingly, there are no nes and no scope for private enforcement where parties seek remedies for an ‘infringement’. The remedies which are imposed may be structural or behavioural and can result in considerable burdens for businesses. Moreover, the system has become more legalistic as a result of the availability of, and frequent resort to, judicial review of all aspects of the process before the Competition Appeal Tribunal. Most importantly, market investigations have had an impact on vast areas of everyday life and society in the United Kingdom, and continue to play a signi cant role in ensuring that consumers bene t from effective competition in markets. The chapter will outline the historical development of market investigations, then provide an overview of the institutions and processes. The CMA’s role in undertaking market studies and market investigation references will precede a discussion of the market investigation and remedial process. There will be a discussion of examples of the experience in undertaking market investigations followed by consideration of judicial review by the Competition Appeal Tribunal.