In this talk an a-posteriori error analysis for optimal control problems is discussed. In particular, the error analysis is applied to reduced-order Galerkin approximations for the optimal control problems. In particular, proper orthogonal decomposition (POD) is utilized to derive reduced-order approximations. Using the error analysis one can estimate the difference between the suboptimal (POD) solutions and the associated exact (but unknown) optimal solutions. Numerical examples illustrate the realization of the error analysis for certain applications.