A comprehensive dynamic analysis of soil-structure systems is the most demanding task in realistic design of earthquake resistant structures. The two main facets of the analysis are the appropriate input parameters with suitable foundation model and the ground motion. The large computational effort and time required for the foundation analysis makes the choice of foundation model very important. Further, the uncertainties involved in defining the design ground motion representing the nature of earthquake shaking, appropriate for the site, make the problem all the more diffioult. In evaluating the response of structures to earthquakes, it is normally assumed that the motion which is experienced by the foundation of a structure is the same as the free field ground motion. This approach is strictly valid only for structures supported on rigid base. The foundation motion in such cases can not possibly be influenced by the motion of the super- imposed structure and the response of the structure can be evaluated correctly by the use of free field ground motion as the foundation motion.