The use of payment services, which is one of the most important banking products, requires co-operation between banks and therefore it presents a number of peculiarities to be taken into account when assessing the antitrust impact of this kind of co-operation. A relevant aspect related to the trade-off between co-operation and competition involves the definition of a common Multilateral Interchange Fee (MIF) by means of which services provided by banks involved in a card payment transaction are remunerated. The determination of a MIF by the banks corresponds to a self-regulation process within the payment system, which is currently accepted by various competition and regulatory authorities, even if the MIF constitutes a restriction of competition under antitrust rules. The authorities limit the negative impact of the MIF on competition by imposing specific conditions provided for by the law; the most important of these concerns the imposition of an objective and cost-based analysis for determining the interchange fee, which should be subject to periodical revision in light of the growth in the volume of payments.