The response of macromolecular binary mixtures to shear flow is rather complex. Experimental evidence suggests that both shear-induced mixing and shear-induced demixing, observed as suppression or enhancement of concentration fluctuations, respectively, can occur (Hobbie et al, 1994; Larson, 1992; Takebe et al, 1989), depending on the amount of shearing and the molecular characteristics of the polymers (molecular weights, glass transition temperature, Tg). Some of the important open problems on the field are: (i) what happens with real industrial blends; (ii) what is the effect of very high shear rates (200 S−1 and above); (iii) how does the presence of glassy components and their large viscosity difference interplay with the shear influences on the phase characteristics.