The blending of immiscible polymers is an economically attractive route to develop new materials that combine the desirable properties of more than one polymer without investing in new chemistry. The final properties of the blend are strongly influenced by the size, orientation and type of the generated microstructure which is in turn determined by the fluid properties and the flow history. For dilute blends, consisting of Newtonian components, the rheological behavior and morphology development is relatively well understood. Most of the theories and experiments that contributed to the understanding of the morphology evolution in such blends under well-defined flow conditions are summarized in recent reviews by Ottino et al. (1) and Tucker and Moldenaers (2). In this chapter, these reviews will be extended by incorporating phenomena that pertain to the morphology development in more realistic blend systems. First of all, the structure-rheology relationship in compatibilized blends will be addressed. This is clearly of major technological importance since compatibilizers are the main driving force for a future growth of the blend market (3). Secondly, since polymers are viscoelastic materials, effects of elasticity of the components on the morphology development are obviously of interest. Since the previous reviews mainly focused on the connection between theory and experiment - which is clearly easier for Newtonian components - the effect of component elasticity has not been covered extensively. Nevertheless, if one wants to understand the morphology development in realistic systems, component elasticity should be taken into account. Finally, most publications dealing with the relationship between rheology and morphology are restricted to rather dilute systems. This is caused by the difficulties involved in studying - both theoretically as well as experimentally - droplet interactions that affect deformation and breakup phenomena at higher concentrations. Recently however, progress has been made in relating the rheological response to the underlying morphology in more concentrated systems and thus the subject merits attention in this chapter.