Provisional chapter ; Fullerene-based organic photovoltaic devices (OPVs) have attracted increasing interest as a low-cost, lightweight, flexible and easy to process alternative to the silicon-based solar cells. Currently the most efficient fullerene-based OPV devices, although improving dramatically in the last few years, are still a long way from a technologically useful efficiency. The performance of current OPV devices is assumed to exist due to the phase separation of the donor (D) and acceptor (A) materials into co-continuous networks of D and A rich phases, forming the so-called Bulk-Hetero-Junctions (BHJ). A typical BHJ active layer in an OPV device is a thin film mixture of a conjugated donor polymer such as regioregular poly(3-hexyl-thiophene) (P3HT), and a functionalized acceptor fullerene such as [6,6]-phenyl C61-butyric acid methyl ester (PCBM). The morphology of these BHJ films has proven to be extremely important in determining the optoelectronic properties of the corresponding devices. However, the correlations between the structure of these donor/acceptor materials from atomic to microscopic length scales, the resulting optoelectronic properties, and the device performance are still not understood from first principles. The understanding and manipulation of this polymer-fullerene morphology has therefore been the focus of intense electronic and optoelectronic polymer-device research, using a broad range of techniques, yet the results from these studies are less than definitive and, in some cases, contradictory. In this chapter, some of these most relevant studies in the understanding and manipulation of the polymer-fullerene morphology are reviewed.