Copper and copper aluminum (2 and 6 wt% aluminum) with two orientations, [001] and [134], were subjected to high intensity laser shocks (energy levels of 40-300 J; energy densities of 15-70 MJ/m(2) and durations of 2.5 ns). The defects created were characterized by transmission electron microscopy. The slip-twinning transition was determined quantitatively in terms of both orientation and stacking fault energy. The threshold twinning pressure for copper oriented to [001] decreases with decreasing stacking fault energy from 40 GPa for pure copper to less than 20 GPa for copper-2 wt% aluminum. For the [134] orientation, pure copper twinned at pressures on the order of 60 GPa, and whereas the copper-2 wt% aluminum alloy readily twinned at pressures less than 40 GPa. The results are rationalized in terms of a criterion in which slip and twinning are considered as competing mechanisms. A constitutive description using a modified MTS (mechanical threshold stress) constitutive equation applied incorporating slip and twinning in terms of orientation, stacking fault energy, temperature rise due to shock heating, and strain rate. The predictions are in agreement with experiments. The constitutive description provides a rationale for the experimental results; the calculated thresholds for [001] and [134] are, respectively: 17 GPa and 25 GPa for pure copper, 9 GPa and 13 GPa, for Cu-2wt % Al and 1 GPa and 2 GPa for Cu-6wt% Al.