The discussion in this paper examines the emergence of what may now be fairly described as the ‘European cartel offence’: that species of infringement of Article 81 of the EC Treaty, increasingly referred to as ‘hard core cartel’ activity, connoting real delinquency which justifies the imposition of severe penal sanctions. This development is significant not just as a matter of more explicit regulation within the field of competition law, but also as part of a wider context of vilification and criminalisation of certain well-established forms of cartel behaviour. As national systems (such as that in Britain) have introduced new criminal offences in relation to cartels, it is illuminating to consider and compare the evolution of the supranational ‘European cartel offence’. Having its basis in the infringement of Article 81 of the EC Treaty, the offence is necessarily one of collusion, being an agreement or concertation for anti-competitive purposes. As such, the more specific nature of the ‘offence’ has been established incrementally, very much as the product of argument presented in appeals relating to issues of evidence and penalties. It has been necessary to consider whether the offence resides in the planning or the implementation of anti-competitive activity, or both, and whether it comprises specific acts or a continuing pattern of behaviour. The mature version of the ‘offence’, worked out in the jurisprudence of the European Commission and the Community Courts, is based on the organising concept of ‘the cartel as a whole’. This process of forging a ‘European cartel offence’ provides an instructive lesson in the legal construction of criminality and the resort to a form of organisational responsibility. It also points to the emergence of a bifurcated system of both individual and corporate liability in this context.