Current methods of SSI analysis can be classified according to the choice of surface Q. In “direct” method they try to put Q on some physical boundaries in the soil, usually apart from the basement. In “impedance” method they put Q right on the soil-structure contact surface, additionally presuming the rigidity of this surface. In “combined” method they put Q on the boundary of the “modified volume” (sometimes Q is a flexible soil-structure contact surface, but sometimes there exists some additional soil volume around the basement modified with the appearance of structure). Let us discuss some details of these methods one by one.