Abstract
The revelation principle states that it is without loss of generality to restrict attention to direct mechanisms and, consequently, that incentive compatibility is necessary for implementation. This paper extends the discussion beyond Bayesian Nash Equilibrium by providing sufficient conditions on the solution concept that ensure any implementable social choice function can be implemented via a direct mechanism. These conditions do not generally imply incentive compatibility is necessary for implementation, as the class of solution concepts requiring incentive compatibility for implementation is characterized via a logically independent condition.