There were three objectives of the soil-structure interaction (SSI) project of the Seismic Safety Margins Research Program (SSMRP). They were (1) to model SSI for systems analysis, using state-of-the-art analysis techniques; (2) to identify important parameters in the SSI phenomena through sensitivity studies; and (3) to compare analysis techniques. SSI was modeled in the systems analysis by the substructure approach, as implemented in the CLASSI family of computer programs. The CLASSI formulation clearly separates the roles of earthquake, soil, and structure--a basic requirement of the systems analysis. The calculative process is extremely efficient, as it must be to perform repeated deterministic analyses simulating earthquake occurrences. The SSI input to the system analysis is detailed. SSI analysis of the Zion Nuclear Power Plant was examined in relation to modeling decisions concerning the free-field ground motion and idealizing the soil-structure system.