Shock hardening of Cu-8.7Ge has been shown to depend strongly on both shock pulse amplitude and shock pulse duration. Because of the time dependent nature of plastic deformation processes, the strengthening effects associated with changes in pulse duration at a constant pulse amplitude pass through a maximum at ∼0.07 μs, followed by an increase to the same high level for durations above lμs. The maximum at short pulse durations arises from twin boundary strengthening, which is related to the development of extremely fine twins with a large amount of twin boundary area per unit volume of twinned material. For longer durations, the twins increase in thickness with less twin boundary area, thus decreasing the strengthening contribution. At the same time, the strengthening from dislocation substructures increases monotonically over the entire range of pulse durations. Estimates of the dislocation generation rate indicate that it is at least 1021 - 1022/m2/s for the conditions studied. Also, it has been found that twins form in Cu-8.7 Ge in less than 0.0lμs. Finally, it has been established that very short duration shock pulses provide an important new technique to study the time dependence of plastic deformation processes.