During the lifespan of real structures, including building structures, bicycles, or automobiles, they are constantly subject to time-dependent excitations. A major piece of work on dynamics of structures is The Theory of Sound by Lord Rayleigh. In terms of civil engineering applications, the structural dynamics of trusses and frames was considered by H. Reissner, of bridges by F. Bleich, and of buildings by G. Alfani. Structural dynamics can be, at least, classified into a steady harmonic vibration from a conservative force and a non-steady vibration from non-conservative forces. If an asymmetric structure is excited under earthquake motions, torsional response will set in as well. If the structure is set in motion after an initial excitation, like a hammer blow, the structure will oscillate. This chapter considers a structure subject to ground shaking induced by earthquakes. Real ground acceleration induced by earthquakes does not appear to have any clear pattern, and the resulting structural response is consequently not easy to interpret.