Mergers can give rise to two types of anticompetitive effects: unilateral effects and coordinated effects. The latter arise if after merger firms can increase their market power by coordinating their actions. In this chapter we explain what coordinated effects are and how they can be identified. As building blocks for the analysis of coordinated effects in mergers, we review the economic meaning of collusion, and assess the factors that allow firms to reach and enforce collusive outcomes. We also review some approaches for quantifying coordinated effects, and provide an overview of the use of coordinated effects in European merger control.