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As mentioned in a companion report, 1 there are at least two ways in which competition may be threatened, other than by single dominant firms. A first situation is when market concentration is high enough for non-competitive outcomes to result from the individual profit-maximising responses of firms to market conditions (from what can be called “individual rivalry”, in other words); in such a situation, firms may be able to exert some market power, even when none of the firms would be considered individually dominant. The second way in which competition may be threatened is when a number of firms engage in what economists refer to as tacit collusion, 2 as a result of which their behaviour may approximate that of a single dominant firm; tacit collusion has been dealt with under the notion of collective dominance in a number of important Court decisions and corresponds to the “coordinated effects” studied in the US. Our companion report proposes an overview of these two threats to competition and of their implications for merger control, as well as a detailed economic analysis of the impact of mergers on market power in oligopolistic industries (Section II) and of the quantitative approaches