The Structure In Media (SIM) Code determines the response of a structure, embedded in soil/rock media, to a specified dynamic disturbance in the media. The structure is modeled as a series of lumped mass, elastic beams which may be interconnected with elastic springs. The disturbance in the free field is specified in terms of an accelerogram, the scale of which may vary with depth. Soil/structure interaction is treated in two parts. The first determines the interaction forces developed at the base of the structure while the second evaluates the interaction forces developed along the side walls of the structure. Details of the model are discussed.