Abstract. Seismic fragility is the main tool used in the performance-based earthquake engineering, and represents the probability of a dynamic system to enter a damage state for a given intensity measure (IM). The most commonly-used seismic IMs are the peak ground acceleration (P GA), which is the absolute maximum of the ground acceleration, and the spectral acceleration (SA), which characterizes well the response of simple linear systems, but shows large uncertainties in the characterization of the seismic response of complex non-linear systems. The main assumption that underlies the basis of the selection of P GA or SA as IMs is that they correlate satisfactorily with the demand parameters (D) of realistic structures, i.e., nonlinear, complex dynamic systems. Statistics and tools of random vibrations are employed to quantify the dependence relations between seismic IMs and structural demand parameters, for non-linear systems subjected to seismic ground-motion records.