The nature of rheology is such that one can only measure indirectly the flow and deformation of a material in order to characterize the nature of its non-Newtonian behaviour. This leads naturally to a broad range of inverse problems including relatxation modulus, relaxation spectrum
and molecular weight distribution recovery. The success of any recovery depends heavily on the formulation of the model that relates the available measurements to the phenomenon of interest. This will be illustrated in terms of how the fading memory of the relaxation modulus of linear viscoelasticity is modelled.