The UK Cartel Offence was introduced in the Enterprise Act 2002 to challenge hard-core cartels and enhance the deterrent effect of the UK competition regime. In its initial phase of operation there was some success. However, a number of significant cases failed to secure convictions. This damaged confidence in the ability of the UK competition authorities to bring successful prosecutions, and ultimately questioned the usefulness of the Cartel Offence. This Chapter examines the problems that beset the original Cartel Offence and the lessons learned from the small number of prosecutions brought before the courts. It goes on to examine the reforms in 2013, that removed the controversial ‘dishonesty’ element from the offence, and replaced it with carve outs for openness and publication. Alongside the practical issues in relation to the development of the UK Cartel Offence consideration is also given to a parallel process which saw a form of consensus developing in the academic literature as to the nature of the wrong at the heart of individual cartel activity. It is suggested that this greater understanding can be used to direct efforts to rebuild confidence in the reformed UK Cartel Offence going forward. Increased importance should be given to the securing of good evidence of individual culpability in relation to cartel activity during the investigation phase. Once good evidence is secured, better prosecution cases can be built on the basis of the new narrative of wrongfulness for hard core cartel activity