Competition law has changed considerably in the EU and UK in the 23 years since the frst edition of this student textbook. Even in the seven years since the ffth edition, in addition to extensive new enforcement practice, soft law and case law, we have witnessed further considerable legislative reforms at both the EU and UK levels. In particular, the 2016 UK referendum, the majority for ‘Vote Leave’, and the subsequent Brexit/ withdrawal of the UK from the EU had a signifcant impact on the substantive scope of the EU and UK competition law rules and the roles of the various institutions involved. This edition of the textbook brings all these changes together to provide a focused ‘one-stop shop’ learning platform for undergraduate students taking a competition law module in any UK university. Following Brexit, given that the UK is no longer a Member State of the European Union, there is inevitably greater focus on the UK rules and enforcement practice, but is crucial to stress that knowledge and understanding of EU competition law remains vital to any student and practitioner of competition law based in the UK for several reasons. First, EU competition law continues to apply to any UK businesses trading in EU markets. Secondly, practitioners may be advising international clients in relation to their activities in more than one jurisdiction, including the EU. Thirdly, the main UK competition law prohibitions are likely to continue to be interpreted consistently with the equivalent EU competition law rules. Finally, EU competition law, including its policy development by the European Commission and jurisprudence by the Court of Justice, continues to be very infuential on competition law and policy developments in other legal systems.