Abstract: In the common agency problem multiple mechanism designer simultaneously attempt to control the behavior of a single privately informed agent. The paper shows that the allocations associated with equilibria relative to any ad hoc set of fessible mechanisms can be reproduce as equilibria relative to (some subset of) the set of menus. Furthermore, equilibria relative to the set of menus are weakly robust in the sense that it is possible to find continuation equilibria so that the equilibrium allocations persist even when the set of feasible mechanisms is enlarged.