We follow the perturbations in the flow of interstellar gas which result from steady forcing by spiral gravitational fields of various strengths. The density response is quite nonlinear even if the amplitude of the spiral field maintained by the disk stars is only a small fraction of the basic axisymmetric field. An analytical study of the properties of slightly nonlinear flows yields certain results which are qualitatively similar to those found numerically for fully nonlinear flows. Galactic shocks arise naturally, indeed necessarily, if the strength of the underlying spiral gravitational field exceeds a certain critical value. The breadth of the zone of high gas compression depends critically on whether the Doppler-shifted phase-velocity of the stellar density wave is greater than or less than the “effective acoustic speed” of the gas. In the former case, very narrow compression zones result; in the latter, quite broad zones. This distinction may explain why some galaxies have narrow optical arms while others have broad optical arms. In addition, a certain range of values for the intrinsic frequency of the wave gives rise to ultra-harmonic reasonances which can introduce secondary compressions of the interstellar gas. This result may relate directly to the origin of the Carina spiral feature in our own Galaxy as well as to the phenomena of branches, spurs, and feathers which are often seen in external spiral galaxies.