This paper was prepared as background for a discussion held at the OECD in June 2015 on oligopoly markets. It focuses on collusion and conduct facilitating collusion in oligopolies. It provides a brief introduction to the notion of agreement, covers merger control and the analysis of the co-ordinated effects of mergers, the use of abuse of dominance or monopolisation-type provisions and additional tools that a number of jurisdictions have adopted to address potential enforcement gaps.