The large mass method is used to obtain the total dynamic response of structures using excitations prescribed at base points. This paper presents the concepts and application of the method derived and used in the analysis of Space Shuttle Main Engine (SSME) structures. The theory and limitations that make the method work are discussed first. The effect on the system responses, including the addition of nonelastic (rigid body and large mass) modes due to the presence of the large masses, is then covered. The resonance concept leads to the establishment of criteria to define the analytic limit of the excitation frequency. The significance of the pseudostatic component and the relationship with the nonelastic modes are investigated in a simple system. The study shows that the frequency ratios, system properties, and the similarity between excitations are key factors to make the pseudostatic term more prominent. Examples with sinusoidal excitation are used to demonstrate the concepts. The method is finally applied in the SSME structures with excitations compiled from engine test data. Results are compared to further demonstrate the concepts and define the future needs for improved test data.