This briefing sets out detailed analysis of the landmark Supreme Court judgment in Merricks v Mastercard, which largely confirmed the less restrictive approach to certification set out by the Court of Appeal when it overturned the Competition Appeal Tribunal’s (CAT’s) original refusal to grant the Collective Proceedings Order (CPO) sought by Mr. Merricks in respect of a £14bn opt-out competition collective action brought against Mastercard (in respect of losses alleged to have resulted from the use of anti-competitive multilateral interchange fees). We address each of the errors of the law which the Supreme Court considered had been made by the CAT when it refused to certify the claim, as well as considering the practical implications of the judgment for future CPO applications and identifying some important questions which remain unresolved. In particular, we consider the potential impact of the “relative suitability” approach endorsed by the Supreme Court, and the consequences of the judgment for the process to be followed at future certification hearings, including the possibility of cross-examination of expert witnesses.