A robust algorithm for simulation-driven design optimization of microwave structures evaluated using CST MICROWAVE STUDIO® is described. The algorithm exploits gradient information obtained using adjoint sensitivity (if available) or finite differentiation. It also uses trust region approach that ensures good convergence properties and improves the overall performance. The efficiency of our approach is demonstrated using several examples of microwave structures. We also discuss an extension of the algorithm where the sensitivity of complex-valued responses rather than of the real-valued ones is used. A performance comparison with other optimization techniques is also provided. © 2011 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2012.