The object of this article is to discuss and compare the role that different constituencies play in U.S. 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 U.S. procedures differ in terms of their basic design and in terms of the procedures that are naturally associated with these alternative designs.