The problem of the dynamic response of a structure to earthquake ground motion has been formulated in a manner which permits separation of the characteristics of particular structures from the characteristics of the earthquake. The expression involving the characteristics of the earthquake is defined as the “spectrum” of the earthquake and it is shown that the spectrum is a plot of the maximum response of a simple oscillator versus the period of the oscillator. Eighty-eight such spectra were computed by means of an electric analog computer and are presented in this paper. It is found that damping is a very important parameter in the over-all problem; relatively small amounts of damping reduce the structural response sharply. Further research on damping in buildings is recommended, and it is also proposed that the spectrum be used as a quantitative measure of earthquake intensity.