In this paper, a novel parameterization of all decentralized stabilizing controllers is employed in mathematically formulating the best achievable decentralized performance problem as an infinite dimensional optimization problem. Finite dimensional optimization problems are then constructed that have values arbitrarily close to this infinite dimensional problem. An algorithm which identifies the best achievable performance over all linear time-invariant decentralized controllers is then presented. It employs a global optimization approach to the solution of these finite dimensional approximating problems.