A new critical excitation method is developed for soil-structure interaction systems. In contrast to previous studies considering amplitude nonstationarity only, no special constraint of input motions is needed on nonstationarity. The input energy to the soil-structure interaction system during an earthquake is introduced as a new measure of criticality. It is demonstrated that the input energy expression can be of a compact form via the frequency integration of the product between the input component (Fourier amplitude spectrum) and the structural model component (so-called energy transfer function). With the help of this compact form, it is shown that the formulation of earthquake input energy in the frequency domain is essential for solving the critical excitation problem and deriving a bound on the earthquake input energy for a class of ground motions. The extension of the concept to MDOF systems is also presented. (c) 2005 Elsevier Ltd. All rights reserved.