Transient burning behavior of solid propellants often occurs under a rapid pressure excursion and is caused by the finite relaxation times required for the solid and/or gas phases to adjust their temperature profiles.The instantaneous burning rate under transient conditions may therefore d i f f e r significantly from the steady-state value corresponding to the instantaneous pressure.The purpose of this chapter is: 1) to report the state-of-the-art on transient burning studies; 2) to explain the detailed mechanism of transient burning; 3) to classify the existing theories to facilitate more direct comparison; 4) to point out the limitations of each model and its general validity; and 5) to summarize the important experimental observations and theoretical results.Various studies indicate that the transient burning e f f e c t is high for higher values of pressurization rates, temperature sensitivity, surface heat release, surface activation energy, and burning rate exponent and lower values of propellant initial temperature and strand burn rate. Nomenclature A = pre-exponential factor in Arrhenius equation a = burning rate constant in steady burning relation, r 0 =a(p) n