The objective of this paper is to discuss and compare the role that different constituencies play in US and EU procedures for merger control. We describe the main constituencies (both internal and external) involved in merger control in both jurisdictions and discuss how a typical merger case would be handled under these procedures. At each stage, we consider how the procedure unfolds, which parties are involved, and how they can affect the procedure. Our discussion reveals a very different ecology. EU and US procedures differ in terms of their basic design and in terms of the procedures that are naturally associated with these alternative designs. On the one hand, there is a single investigator and decision maker operating under a symmetric mandate in the EU and on the other hand, an investigation and settlement operating under the threat of a court decision in case of challenge only in the US. The EU has developed numerous procedures and has granted extensive rights to the parties in the context of these procedures in order to provide some guarantee that the Commission’s role as investigator and decision maker at first instance is not abused. By contrast, the US procedures appear to be rather informal, the balance in the investigation and evaluation of the merger being provided by the credible threat of a court decision. With a strong federal government that has extensive competences for regulation, merger control on competition grounds is subject to the additional public interest test of regulators in the US. Such additional control is weak in the EU, which has more limited competences for regulation. In addition, both the executive and the legislative powers are more fully developed at the federal level in the US. Both the executive power and the legislative seem to be in position to wield greater influence on enforcement in the US than is the case in the EU.