Chapters 4 – 6 detail the surrogate modeling technique in MDO. This chapter introduces the basis of realizing optimization design—sensitivity analysis. Sensitivity analysis (SA) was first used in control system design (Radanović L. Sensitivity methods in control theory: proceedings of an International Symposium held at Dubrovnik, August 31–September 5, 1964. 1st ed. Oxford New York: Symposium Publications Division, Pergamon Press; 1966 [1]) to analyze the effect of variations in model parameters on system performance. In MDO, SA is used to measure the sensitivity of system performance to variations in design variables or other parameters. By conducting a sensitivity analysis of the vehicle system performance with respect to design variables or other parameters, the impact of the system design variables and parameters on the objective function or constraint function can be determined. Then the design variables can be selected, and the system parameters that need to be focused on are identified. The sensitivity information can also be used to aid in determining the optimization search direction during the optimization process. Therefore, a robust, efficient, and accurate sensitivity analysis method with broad applicability is essential in studying MDO theory for flight vehicles.