This chapter discusses the Competition Act 1998, which serves as a basis for UK competition law. Following an overview of the Competition Act, it considers decisional practice and case law under the so-called Chapter I and Chapter II prohibitions in the Competition Act, which are modelled after Articles 101 and 102 TFEU. It discusses the relationship between EU and domestic competition law, including the important ‘governing principles’ clause in section 60 of the Competition Act, which is intended to achieve consistency with EU law. The chapter contains a table of all the decisions under the Competition Act to have been published on the website of the Competition and Markets Authority (‘the CMA’) since the eighth edition of the book up until 8 December 2017. The chapter concludes with a discussion of the application of the Competition Act in practice and the possible implications of Brexit for UK competition law.