This chapter considers propellant combustion under the conditions of periodically varying pressure. The importance of such processes is warranted by the necessity to understand the reasons behind deviations of actual burning regimes in solid rocket engines from designed steady-state regimes. Among observed nonsteady effects, the are soft or hard (triggering) excitations of burning rate and pressure oscillations. Widespread usage of solid rocket engines proves that there are practical empirical methods of eliminating these unwelcome effects which normally appear early in the design stage.