A unified assessment of elastic and inelastic response spectra is presented. The effects of various system and excitation parameters on spectral response are investigated. Different spectral forms such as strength spectra, ductility reduction spectra, and damping reduction spectra are employed as graphical tools in the analytical evaluation. The applicability of the expressions for elastic and inelastic response spectra that are employed in seismic design codes is tested by using an ensemble of 21 earthquake accelerograms, all recorded on firm ground along the west coast of North America. New expressions are derived in order to evaluate the coupled effects of damping and ductility ratios on inelastic response spectra.