The output of ensemble Ensembleoptimization method history-matching methods is an ensemble of model realizations approximating the posterior distribution conditioned on historical data. The goal is to use this ensemble representation of the reservoir uncertainty when planning future production and drainage strategies. In closed-loop reservoir management workflows, robustOptimizationrobust optimization methods are now a key component. These methods involve optimizing the expected value of future controls and production strategies over the ensemble of history-matched models. The “robustness” results from the explicit accounting for the reservoir uncertainty in the optimization. In this chapter, we will discuss a robust optimization method, EnOpt, where we define the optimum as the controls and strategies that result in the highest net present valueNet present value when averaged over the ensemble of reservoir models.