A constitutive model for quasicrystal plastic deformation is presented. The constitutive description incorporates an order parameter that accounts for deformation-induced disordering and thermally activated reordering. An evolution equation for the order parameter constitutes part of the model. Model predictions, such as the calculated dislocation density evolution and the attendant stress-strain behaviour, are compared with experimental data. A good accord between the calculations and the experiment gives us the confidence that the model captures the salient features of quasicrystal plasticity.